`Buffer` is not usable as a trait object

[lifthrasiir]

let mut buf = std::io::BufReader::new(b"asdf");
let _: &mut Buffer = &mut buf; // error

As a side effect of #17704 (and I think @huonw has noted this possibility correctly), std::io::Buffer is no longer usable as a trait object. We need a separate trait and a blacket implementation for it.

[alexcrichton]

cc @aturon

[aturon]

Indeed, this seems to've been missed in the original rollout. As usual, this will actually make the default methods more widely usable.

We need to develop a convention around these extra blanket extension traits.

[taralx]

Is it actually unsafe to use Self in the return type?

[lifthrasiir]

@taralx It wouldn't make sense from the perspective of the caller. The caller cannot see the concrete type that makes the return value up (T<Self> and T<Trait> are quite different). A proxy that converts the return value to the trait object may work, but it is surprising and it still doesn't work for every Self-typed return value (e.g. iterators).

[aturon]

This has now been addressed.