component model: async host function & embedding support

[pchickey]

Based on #5065.

This PR adds the async suite of variations on Linker::wrap_func, Func::call, TypedFunc::call, and the various instantiate functions, for use in async Rust contexts.

This ends up being pretty simple: all of the async store machinery from core wasmtime is straightforward to reuse here.

I created a new test suite at tests/all/component_model/async.rs to smoke test the new functionality.

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[alexcrichton]

I've double-checked bits and pieces internally and I think this should generally be good to go. I'm always somewhat wary of async since it's got unsafe underpinnings with fibers and whatnot but I'm at least for now confident enough in this that I'd be happy to merge.

[pchickey]

Thanks! I just need to write some tests.

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