Add AsyncWrite implementation for IntoAsyncRead

[najamelan]

This implements AsyncWrite for IntoAsyncRead when the inner stream implements it,
forwarding all method calls.

This allows working on Types that keep both impls, and allows framing
them with codecs where those require both traits (like tokio codec).

r? @cramertj

[cramertj]

This seems like a fine change to me, but did you mean to have a different commit message? The commit message and PR name seem to suggest a different thing than what is implemented here.

[najamelan]

@cramertj We can change the commit message to whatever you like. From the user perspective it's like you have a Type which implements AsyncWrite and when you call into_async_read, you lose the AsyncWrite functionality. That's why I called it like this. This doesn't actually provide any new functionality, just makes the inner AsyncWrite available on the wrapper to avoid losing the functionality, and to be able to work with API's that require both. Especially since IntoAsyncRead will consume the inner stream.

Which part does not correspond to the code? I'm a bit confused. It looks like the first paragraph describes the implementation exactly.

[taiki-e]

"Add AsyncWrite implementation for IntoAsyncRead" or "impl AsyncWrite for IntoAsyncRead" seems good commit message to me. (refs: #1575, #1503)

[najamelan]

I changed the pull request title and the commit message.