AsyncWrite support

[Alexei-Kornienko]

Would be good to add impl of FrameEncoder/FrameDecoder where W: AsyncWrite (futures::io::AsyncWrite), R: AsyncRead

This would enable user the possibility to use compression in async data streams

[PSeitz]

Thanks for the issue, yeah I also thought about that. It would be nice to have, but I'm not sure how good it can be managed to have both in terms of code duplication.

[Alexei-Kornienko]

One solution that I could propose is:

1. Refactor implementation to always work with AsyncWrite trait
2. Add a thin Wrapper that would take any type that implement Write and impl AsyncWrite for it without returning Poll::Pending

In result Async version would be used as is.
Sync version could internally use block_on (https://docs.rs/futures/0.3.15/futures/executor/fn.block_on.html) to convert sync call to async call

Pros:

• only 1 impl of the algorithm without copy/paste
• for sync version compiler should be able to optimize out all await keywords and make it zero cost (need to benchmark)

Cons:

• hard dependency on futures crate
• slightly more complicated code using async/await syntax

[arthurprs]

One significant problem right now is that the ecosystem is split regarding AsyncRead/Write. Every runtime has an its own (tokio, ..) set of traits and the futures crates have another one.

[PSeitz]

A good idea may be to add lz4_flex support to https://github.com/Nemo157/async-compression