`cargo watt` subcommand

I made a cargo subcommand called [cargo watt](https://github.com/jakobhellermann/cargo-watt), which aims to improve the tooling listed in the README.

Currently, it does two things:
1. Compile a proc-macro crate (locally, from git or from crates.io) and generate a crate which exposes the compiled wasm
2. Verify that that wasm file was compiled from some source

Initially I wanted to replace
```rust
#[proc_macro]
pub fn the_macro(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
/* ... */
}
```
with
```rust
#[no_mangle]
pub extern "C" fn the_macro(input: proc_macro2::TokenStream) -> proc_macro2::TokenStream {
the_macro_inner(input.into().into()
}
pub fn the_macro_inner(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
/* ... */
}
```
but that doesn't work because [Into does not exist in wasm](https://github.com/dtolnay/watt/blob/master/proc-macro/src/lib.rs#L50).

So my solution was to just replace the TokenStreams and hope for the best, while also using a [patched version of syn](https://github.com/jakobhellermann/syn-watt) which basically just has all instances of `proc_macro` replaced with `proc_macro2`.

This actually works for quite a few crates (e.g. `serde-derive`, `typed-builder`, `tracing-attributes`) but it doesn't for some other ones (everything depending on `synstructure` and some others).

Now I am wondering:
Is it fundamentally impossible to provide the `From`-impls because `proc-macro` just doesn't exist in wasm, or would it be possible to do this transformation in some other way?

Start with proc-macro/src/lib.rs at the cited conversion area, then compare the cargo watt behavior with the patched syn-watt repository and the listed failing crates. Determine whether the requested conversions can be supported in wasm or require a different transformation; done means reaching a reproducible conclusion about the approach.

Written by the indexing model from the issue text.