Convert async iterators to streams

[olanod]

An initial implementation of what I proposed in #2399
It would be nice to have the ability to turn async iterators into Rust streams in the same way we can turn promises into futures. Instead of creating a separate crate for this utility I think it fits well in the scope of wasm-bindgen-futures.

[alexcrichton]

Thanks for this! I'm a bit hesitant to do this at this time though since it introduces futures-core as a public dependency. AFAIK Stream is intended at some point to make its way into the standard library, right? If so perhaps this could be gated behind an off-by-default Cargo feature?

[olanod]

Yeah it looks like rust-lang/rust#79023 could land anytime which would also remove the need for the dev-dependency. I'll put this under a stream feature if that's ok?

[olanod]

@alexcrichton Added the feature under the stream flag even for the test because it seemed I would have to change the version of wasm-bindgen-test all over the place. Also mentioned the existence of feature in the Readme as experimental which can be made "stable" later when there is core::stream::Stream and perhaps also adding the feature of Stream to AsyncIterator(ReadableStream?) conversion.

[alexcrichton]

Looks good, thanks! For a small amount of extra future-proofing, though, could this perhaps be named futures-core-03-stream?

[alexcrichton]

Thanks!