TokenSink trait forces interior mutability/RefCell usage

https://github.com/servo/html5ever/blob/464118468457fde7ade8b511cdfa5588f8a10f41/html5ever/src/tokenizer/interface.rs#L83-L99

The TokenSink trait's process_token method takes &self

Many usecases for a TokenSink are going to involve mutability. This includes multiple files in this repo such as tree_builder.

I think TokenSink would be better like this:

pub trait TokenSink {
    type Handle;
    fn process_token(self, token: Token, line_number: u64) -> TokenSinkResult<Self::Handle>;
    fn end(self);
    fn adjusted_current_node_present_but_not_in_html_namespace(self);
}

Usecases that need mutability could then be impl TokenSink for &mut MyTokenSink, and those that don't like the printer examples can be impl TokenSink for &MyType.
I haven't tried implementing this refactor, it might not actually work out well.

Start with html5ever/src/tokenizer/interface.rs#L83-L99 and compare the TokenSink signature with the tree_builder implementation at the linked location. Check the printer examples and other TokenSink implementations to assess whether taking self or reference receivers works across the repository. Done means the refactor's feasibility and all affected implementations are resolved.