std::vec::raw: convert copy_memory to a method.
std::vec::bytes: remove the reference to overlapping src and dest in
docs for copy_memory.

&mut [u8] and &[u8] really shouldn't be overlapping at all (part of the
uniqueness/aliasing guarantee of &mut), so no point in encouraging it.
std::vec: convert .copy_memory to use copy_nonoverlapping_memory.

It is required that &mut[]s are disjoint from all other &(mut)[]s, so
this assumption is ok.