std::io::{empty, sink} do not replace /dev/null

[aturon]

In particular, splitting up the Read and Write halves mean that you can't use these in a generic context that expects a full stream (Read + Write).

We should consider deprecating these in favor of a single null constructor that is both Read and Write.

[aturon]

cc @alexcrichton

[alexcrichton]

I'm personally comfortable with a io::null name, but if we were to deprecate the empty/sink constructors in favor of this we'd want to basically insta-stabilize it at which point we may want to get some more consensus before taking action.

[steveklabnik]

empty and sink are now stable, so I guess this can be closed?

[alexcrichton]

Yeah I think this better belongs as an RFC issue nowadays and I haven't heard much of a desire for this recently so we may be covered.