reflect: do not cache trivial values in DeepEqual.

DeepEqual caches addresses of compared values
each time it visits addressable values. This is
more expensive than actually comparing them in
the common case of large slices of bytes or integers.

Also add a fast path for slices with identical
underlying array.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13000044