codec: rewrite of codec::Framed (#2368)

Framed was designed to encapsulate both AsyncRead and AsyncWrite so
that it could wrap two-way connections. It used Fuse to manage the pinned
io object between the FramedWrite and FramedRead structs.

I replaced the Fuse struct by isolating the state used in reading and
writing, and making the code generic over that instead. This means
the FramedImpl struct now has a parameter for the state, and contains
the logic for both directions. The Framed* structs are now simply
wrappers around this type

Hopefully removing the `Pin` handling made things easier to
understand, too.

tokio-util/src/codec/mod.rs

@@ -18,6 +18,10 @@ pub use self::decoder::Decoder;
 mod encoder;
 pub use self::encoder::Encoder;
 
+mod framed_impl;
+#[allow(unused_imports)]
+pub(crate) use self::framed_impl::{FramedImpl, RWFrames, ReadFrame, WriteFrame};
+
 mod framed;
 pub use self::framed::{Framed, FramedParts};