Rust: Future/Stream-based "spawn" APIs for WIT streams/futures

[alexcrichton]

@rvolosatovs and @dicej had an idea in a meeting we had today where we should have an API in this crate for converting a Rust Future<Output = T> to a WIT future<T> by sort of "spawning" it, effectively you yield ownership of the future to wit-bindgen crate and that'd give you back a future<T>. That'd then manage polling the original future and then writing the result to the writer half of the channel the runtime provides. Similarly this could work for streams of values (maybe with a bit more finesse to handle chunking)