bitmap: Add first cut at a bitmap module

This adds a module for manipulating bitmaps, based on the API from the
Linux kernel's bitmap.h.  So far it's missing the trickier bits, but is
a little more flexible, allowing bitmaps that don't have to be contained
in an integer number of unsigned longs.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>