Add some initial x86 string optimizations. These make no attempt to use nifty

Add some initial x86 string optimizations.  These make no attempt to use nifty
things like mmx/3dnow/etc.  These are not inline, and will therefore not be as
fast as modifying the headers to use inlines (and cannot therefore do tricky
things when dealing with const memory).  But they should (I hope!) be faster
than their generic equivalents....

More importantly, these should provide a good example for others to follow when
adding arch specific optimizations.
 -Erik