This was supposed to be the foundation for some kind of pre-kernel
environment, but nobody is working on it right now, so let's move
everything back into the kernel and remove all the confusion.
Move the "fast memcpy" stuff out of StdLibExtras.h and into Memory.h.
This will break a ton of things that were relying on StdLibExtras.h
to include a bunch of other headers. Fix will follow immediately after.
This makes it possible to include StdLibExtras.h from Types.h, which is
the main point of this exercise.
