Add first batch of async support

This PR adds a first batch of async support, viz., the async and callback options to canon lift/lower and the supporting canonical built-ins. The PR does not add future or stream, those will be in the next PR (as well as a number of other key parts of the async story; see Async.md#TODO).

There are two commits in the PR, a tiny one that removes something and a big one that adds all the async stuff.

The first commit removes a special case where reentrance was allowed in a very hand-wavy manner for parent-->child-->parent cases. Working through async and thinking about what reentrance even means in an async context, I realized that the hand-wavy approach was wrong and the better way to enable this use case is by adding a new reentrant (name TBD) attribute to function types that basically opts into reentrance at the type-level, allowing parent-->child-->parent reentrance as a special case. I'm pretty sure this won't break any code because it requires doing "donut wrapping" which isn't supported by the producer toolchains yet. Adding reentrant is in the Async.md#TODO list.

The second commit is somewhat big and I'd recommend reading it by:

Start by looking at the additions to Explainer.md and Binary.md, which are pretty small.
Read the new Async.md to get the high-level summary and terminology.
Read the delta to CanonicalABI.md to understand how it all works in detail.

Big thanks to @dicej who has been collaborating on this design and has built a working prototype that we'll now iterate on to precisely match this PR. (Apologies in advance for slow replies over the next week due to in-person wasm CG meeting.)

WebAssembly / component-model

Add first batch of async support #363