These functions have object safety issues. It also has been decided to
avoid vectored operations on the I/O traits. A later PR will bring back
vectored operations on specific types that support them.
Refs: #2879, #2716
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.
Dirkjan Ochtman