Add support for async/streams/futures to Rust generator

[dicej]

This adds support for generating bindings which use the Async ABI along with the stream, future, and
error-context types.

By default, normal synchronous bindings are generated, but the user may opt-in to async bindings for all or some of the imported and/or exported functions in the target world and interfaces -- provided the default-enabled async feature is enabled.

In addition, we generate StreamPayload and/or FuturePayload trait implementations for any types appearing as the T in stream<T> or future<T> in the WIT files, respectively. That enables user code to call new_stream or new_future to create streams or futures with those payload types, then write to them, read from them, and/or pass the readable end as a parameter to a component import or return value of a component export.

Note that I've added new core::abi::Instruction enum variants to handle async lifting and lowering, but they're currently tailored to the Rust generator and will probably change somewhat as we add support for other languages.

This does not include any new tests; I'll add those in a follow-up commit.

This is ready for review, but I'll leave it in draft mode until the following are complete:

• Codegen test coverage for new features in the Rust generator
• Switch to upstream wasm-tools once Add support for async ABI, futures, streams, and errors wasm-tools#1895 is merged and released
• Runtime test coverage for new features in the Rust generator. This requires async/future/stream support in Wasmtime, which I'll soon open a PR for. Since the Wasmtime support might take a while to land, we could consider deferring this item to a follow-up PR.

[dicej]

Oops, forgot to make this a "draft" PR, and GitHub won't let me change it 🤦 . I'll close this and open another one.