Add support for async/streams/futures

[dicej]

This adds support for loading, compiling, linking, and running components which use the Async ABI along with the stream, future, and error-context types. It also adds support for generating host bindings such that multiple host functions can be run concurrently with guest tasks -- without monopolizing the Store.

See the implementation RFC for details, as well as this repo containing end-to-end smoke tests.

This is very much a work-in progress, with a number of tasks remaining:

• Avoid exposing global task IDs to guests and use per-instance IDs instead (and fix no_std build at the same time)
• Track task.return type during compilation and assert the actual and expected types match at runtime
• Ensure all guest pointers are bounds-checked when lifting, lowering, or copying values
• Reduce code duplication in wasmtime_cranelift::compiler::component
• Reduce code duplication between StoreContextMut::on_fiber and concurrent::on_fiber
• Minimize and/or document the use of unsafe code
• Implement {stream|future}.cancel-{read|write} and error-context.{new|debug-string}
• Add support for (Typed)Func::call_concurrent per the RFC
• Add support for multiplexing stream/future reads/writes and concurrent calls to guest exports per the RFC
• Refactor, clean up, and unify handling of backpressure, yields, and event polling
• Guard against reentrance where required (e.g. in certain fused adapter calls)
• Add integration test cases covering new functionality to tests/all/component_model (starting by porting over the tests in https://github.com/dicej/component-async-demo)
• Add binding generation test cases to crates/component-macro/tests
• Add WAST tests to tests/misc_testsuite/component-model
• Add support and test coverage for callback-less async functions (e.g. goroutines)
• Add doc and code comments to new, non-trivial code (especially concurrent.rs and futures_and_streams.rs)
• Switch to back to upstream wasm-tools once Add support for async ABI, futures, streams, and errors wasm-tools#1895 has been merged and released