ConstantTimeCompare is fairly useless if you can't rely on it being zero
when the slices are different, but thankfully it has that property
thanks to the final ConstantTimeByteEq.

Change-Id: Id51100ed7d8237abbbb15778a259065b162a48ad
Reviewed-on: https://go-review.googlesource.com/c/158643Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>