These are easily built off the standard C library functions memcpy
and memcmp.  By marking these inline we stand a good chance of
the C compiler replacing the entire thing with tight machine code,
because many compilers will actually inline a memcmp or memcpy when
the 3rd argument (the size) is a constant value.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>