Where T is a type that can be compared for equality bytewise, we can use
memcmp. We can also use memcmp for PartialOrd, Ord for [u8] and by
extension &str.

This is an improvement for example for the comparison [u8] == [u8] that
used to emit a loop that compared the slices byte by byte.

One worry here could be that this introduces function calls to memcmp
in contexts where it should really inline the comparison or even
optimize it out, but llvm takes care of recognizing memcmp specifically.

The old test for Ord used no asserts, and appeared to have a wrong test. (!).

This should avoid the trait impls showing up in rustdoc.