feat: implement Framed::map_codec

[saiintbrisson]

Motivation

Currently, changing the codec of a Framed is verbose and unnecessarily complicated, as noted in #717.

Solution

This PR implements a method that maps the inner codec to a new type, preserving the read/write buffers and IO state. The syntax was first suggested by Carl Lerche.
The usage is as follows:

let framed: Framed<Io, U32Codec> = Framed::new(io, U32Codec);
let _: u32 = framed.next().await.unwrap();

let framed: Framed<Io, U64Codec> = framed.map_codec(|_codec| U64Codec);
let _: u64 = framed.next().await.unwrap();

[Darksonn]

Thanks! Looks good.

[Darksonn]

You will need to run rustfmt on this.

rustfmt --check --edition 2018 $(git ls-files '*.rs')

[Darksonn]

Thanks.