`impl Read for &Stdin` is missing

[RalfJung]

It is possible to write to a &Stdout thanks to impl Write for &Stdout, but it is not possible to read from a &Stdin as the corresponding impl Read for &Stdin is missing. This seems to be entirely an accident, given that read_line does work on &Stdin.

Cc @rust-lang/libs-api

[joshtriplett]

Seems reasonable to me.

We'd need to FCP the resulting PR, since trait impls are insta-stable.

[0xPoe]

@rustbot claim

[0xPoe]

Sorry, I didn't have time to add it. So see if any other contributors would like to add it. Unassign myself.

[kckeiks]

@rustbot claim

[Dajamante]

@rustbot claim