Implement the new canonical ABI

[alexcrichton]

I've started sending PRs to do this, but I figured it'd also be a good thing to write things down:

• Implement and test NaN canonicalization for float32/float64
• Make a plan for string encodings -- hardcode to utf-8? option-per-language?
• Enable mutability in Rust traits generated for in-wasm since recursive entry is now prohibited (Mutable methods in wit resources #178)
• Use post-return instead of individual free calls
• Update cases in variants to require their type payload
• Add a union type
• Add an option type
• Add an expected type
• Add an enum type
• Add a tuple type
• Add a flags type
• Change return-multiple-values ABI
• Implement MAX_FLAT_PARAMS for passing arguments
• Remove support for old-style *.witx
• Remove support for char8 and usize
• Rename f32 and f64 to float32 and float64
• Change the variant ABI w.r.t. the "join" function
• Split out the string type from list<char>
• Add a bool type
• Add a unit type
• Remove support for multiple return values

(I'll probably add to this over time as I remember things)

[alexcrichton]

Ok I think all the major pieces that can be done now are largely done. The remaining bits sort of need questions and topics on #214 to be settled first. For example post-return-style things aren't done but functions aren't guaranteed to have a post-return so it doesn't make sense for the host generators to implement something with post-return as-is.

[alexcrichton]

I'm going to close this in favor of #314 since the remaining few items here and there that need to be covered by hosts I think require deeper refactorings to fully address or they're inconsequential to not really matter a whole lot.

[dbaeumer]

@alexcrichton I have a question regarding (since all the 'favored' issues are closed):

Enable mutability in Rust traits generated for in-wasm since recursive entry is now prohibited

wit-bindgen version 0.24.0 still generates x(&self) signature in traits. Is this going to change in the future.

[alexcrichton]

It can theoretically yeah but it would be a bit of a nontrivial lift. I didn't handle that during this whole transition due to the complexity involved, but I forgot to open another tracking issue. If you're up for it would you be ok opening a new issue for it?