auto merge of #20910 : sfackler/rust/show-impls, r=alexcrichton

A derived implementation would not be appropriate for the Buffered types
since the buffer is both huge (64k by default) and full of uninitialized
memory. Instead of printing the whole thing, we display how full it is.

I also altered `MultiWriter` to make it generic over Writers instead of
taking `Box<Writer>` trait objects. `Box<Writer>` implements `Writer` so
existing use cases should continue to work, and this enables a more
useful Show implementation in applicable cases.

The change to `MultiWriter` may break code that uses it, but any fixes
should be easy.

[breaking-change]

r? @alexcrichton