Refactor framing to use Streams and Sinks

- Gets rid of `easy` module, instead providing framing support directly
  in the `io` module.

- In particular, adds a framing adapter directly to the `Io` trait,
  which gives you a Stream + Sink object. That object can then be
  `split` into separate `Stream` and `Sink` objects if needed.

- Deprecates the `FramedIo` trait; that's now just Stream + Sink.

- Updates the line framing test to use the stream/sink combinators.

src/lib.rs

@@ -112,4 +112,3 @@ mod heap;
 pub mod channel;
 pub mod net;
 pub mod reactor;
-pub mod easy;