Plan for encoding/simpler prefetching in html5ever

This issue is a collection of some structural changes I want to make to html5ever (and its integration in the servo project).
The intent is both for me to act as a rubber duck and for reviewers to make sure what I plan makes sense (so complaints/questions are welcome).

Current architecture

For simplicity, the following diagram does not include XML parsing, XML is also not really relevant for now.

                              │                                                                              
                              │ Bytes                                                                        
                              │                                                                              
                              ▼                                        ──┐                                   
                        ┌───────────┐    Bytes       ┌─────────────┐     │                                   
               Utf8     │ServoParser├───────────────►│   Decoder   │     │                                   
         ┌──────────────┤(HTML only)│◄───────────────┤Bytes -> Utf8│     │ Servo                                   
         │              └─┬─────────┘    Utf8        └─────────────┘     │                                   
         │                │       ▲                                    ──┘                                   
         │                │       │                                                                          
         │                │       │                                                                          
         │                │       │                                                                          
         │           Utf8 │       │ blocking <script> tags                                                            
         │                │       │                                                                          
         │                │       │                                                                          
         ▼                ▼       │                                    ──┐                                   
┌────────────────┐  ┌─────────────┴──────────────┐                       │                                   
│    Prefetcher  │  │      HTML Parser           │                       │                                   
│                │  │                            │                       │                                   
│   ┌─────────┐  │  │     ┌─────────┐            │                       │                                   
│   │Tokenizer│  │  │     │Tokenizer│            │                       │                                   
│   └───┬─────┘  │  │     └─┬───────┘            │                       │                                   
│       │        │  │Tokens │     ▲              │                       │                                   
│       │ Tokens │  │       │     │              │                       │                                   
│       │        │  │       ▼     │ <script> tags│                       │ html5ever                                  
│       ▼        │  │    ┌────────┴───┐          │                       │                                   
│ ┌─────────────┐│  │    │Tree Builder│          │                       │                                   
│ │Prefetch Sink││  │    └──────┬─────┘          │                       │                                   
│ └─────────────┘│  │           │                │                       │                                   
└────────────────┘  │           │ Tree ops       │                       │                                   
                    │           ▼                │                       │                                   
                    │      ┌─────────┐           │                       │                                   
                    │      │Tree Sink│           │                       │                                   
                    │      └─────────┘           │                       │                                   
                    └────────────────────────────┘                       │                                  
                                                                       ──┘                                   

There are multiple downsides to this architecture:

• The prefetcher blocks the main thread (https://github.com/servo/servo/issues/36482) and tokenizes everything a second time. Tokenization can take a significant amount of time (250ms for https://html.spec.whatwg.org)
• Encoding support with <meta charset> tags is hard, because the decoder is far away from the parser
• Reentrancy requires awkward interior mutability in html5ever, because the TreeSink can invoke the ServoParser again with document.write

Encoding support

When the parser encounters a <meta charset> tag it cannot proceed immediately. Instead, a message needs to be bubbled up from the parser, through the tokenizer to the place where the decoding happens and there we need to change the encoding while parsing. This is why the parser architecture would benefit from the decoder being closer to the tree builder.

Right now the decoding happens in the network_decoder and network_input fields on ServoParser: https://github.com/servo/servo/blob/4e9993128b81b5a3757970786d47fb165ed3ebca/components/script/dom/servoparser/mod.rs#L111-L116.
The problem with naively moving the decoding process into html5ever is that all input would be decoded twice (once in the prefetcher and once in the "main" parser).

The final design plan is to have an (optional) wrapper around the Tokenizer which handles both decoding and buffering of input.
Pretty much all of that is already implemented in https://github.com/servo/html5ever/pull/590 with the DecodingParser type. Note that this type also takes care of the intricacies of document.write which might benefit other users of html5ever. Nico requested that this DecodingParser stay behind a feature flag.

Parallel Parsing / Real Prefetching

My current plan is very similar to https://github.com/servo/servo/pull/19203, except that I want the input stream to live in the parser thread. Otherwise you have to send (read: clone) the input each time you invoke the parser. The input stream is a spec concept that buffers input which has been received from the network but not yet been processed by the tokenizer. It is also where the decoding from bytes to UTF8 happens. html5ever does not currently implement this.

Below is a somewhat simplified diagram. The extra info sent to the parser thread mostly relates to document.write and is not included here for simplicity.

                                                          
 ┌────────────────┐                   ┌─────────────────┐ 
 │  Main Thread   │                   │  Parser Thread  │ 
 │                │      Bytes        │  ┌────────────┐ │ 
 │                ├─────────────────► │  │Input Stream│ │ 
 │ ┌───────────┐  │  (+ extra info)   │  └────┬───────┘ │ 
 │ │ServoParser│  │                   │       ▼         │ 
 │ └────┬──────┘  │                   │  ┌─────────┐    │ 
 │      │         │   Prefetch Ops    │  │Tokenizer│    │ 
 │      │ Parse Op│◄──────────────────┤  └────┬────┘    │ 
 │      ▼         │     Parse Ops     │       ▼         │ 
 │┌─────────────┐ │                   │  ┌───────────┐  │ 
 ││ParseExecutor│ │                   │  │TreeBuilder│  │ 
 │└─────────────┘ │                   │  └───────────┘  │ 
 └────────────────┘                   └─────────────────┘ 

A parse operation in the diagram above could be something like AppendChild or SetQuirksMode - mirroring the methods of the current TreeSink trait.

Notice how this design allows us to support reentrancy without interior mutability in html5ever - the parser thread does not need to know about reentrant parsing at all, since it just processes input from the main thread.

Ordering of changes

The current plan is to

1. Move buffering of input into html5ever. Makes everything else easier - This will be a significant breaking change to the API!
2. Implement parallel HTML parsing in servo, to be able to implement encoding support without decoding everything twice.
3. Implement support for <meta charset>

Start with ServoParser's network_decoder and network_input fields and the DecodingParser implementation described in html5ever pull request 590; then compare the current TreeSink flow with the proposed input-stream and parser-thread design. Done means agreeing on and implementing the planned sequence: move input buffering, add parallel parsing in Servo, and support meta charset handling.