AsyncRead ans AsyncWrite wrapper for EncStream

[bddap]

Implement one or two wrappers for encstream to make it easy to integrate into client programs.

[bddap]

https://docs.rs/futures-preview/0.3.0-alpha.16/futures/io/trait.AsyncRead.html
https://docs.rs/futures-preview/0.3.0-alpha.16/futures/io/trait.AsyncWrite.html