Refactor core wasm host entrypoints

[alexcrichton]

This commit refactors the internals of how guest functions call out to the host. Previously Wasmtime had a split where Func::new-style constructors used one entrypoint and Func::wrap-style entrypoints used a different entrypoint. This was required long ago when we had an array and native ABI calling convention, but nowadays this is no longer required. This refactors things to have a single base-level signature which is the narrow waist through which all other function entrypoints go through. This enables having a single function handle all the things like panicking, call hooks, etc, and everything further builds on top of it.

This commit additionally pushes the async-ness of a function down even further. Previously many *_async constructors would quickly delegate to their non-async counterpart, but this is expected to more-or-less become not the right way to do things as wasmtime moves into the future. Historical refactorings related to GC, for example, have pushed async further down within Wasmtime and this continues that trend. There's no immediate benefit just yet, but this is hoped to make future refactorings easier.

Along the way some minor API changes happened too:

• *_unchecked constructors now work with MaybeUninit<ValRaw> to correctly model how some values may not be initialized.
• Some *_async functions picked up T: Send which should have in theory been required before and are now compiler-required.
• Longstanding/old internal functions have been shuffled around/refactored.

Finally, another eventual goal of this work is to share more between core wasm and components in terms of implementation. Right now the component host function entrypoints are quite different than the core wasm ones and that ideally wouldn't endure forever.