Add WritableStream::into_async_write

[MattiasBuelens]

This adds a way to turn a JS WritableStream into a Rust AsyncWrite.

Note that AsyncWrite passes a borrowed byte slice (&'a [u8]) to poll_write, whereas a WritableStream expects an "owned" Uint8Array. Therefore, we need to copy each given slice into a newly allocated Uint8Array. There's currently no way to avoid this copy, since JS doesn't yet have a concept of "writable byte streams" with BYOB semantics. (When such a concept is added in the future, we should be able to update our AsyncWrite implementation to use such a BYOB writer instead.)

Fixes #9.