This adds support for generating bindings which use the [Async
ABI](https://github.com/WebAssembly/component-model/blob/main/design/mvp/Async.md)
along with the [`stream`, `future`, and
`error-context`](WebAssembly/component-model#405) 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 `stream`s or `future`s 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.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>