New bitwise ops in ck_cc

GCC and compatible compilers ffs/ctz/clz/popcount. Expose that in
ck_cc and default to slow but portable software emulation.
ck_bitmap: more bulk operations, less space, more tests

* new bulk rmw: intersection, intersection_negate (with complement),
* new bulk reads: empty, full, count, count_intersect

* ck_bitmap_iterator fixes: avoid out of bounds read on empty bitmaps,
  and ck_bitmap_next is marginally faster on sparse bitmaps .

* less space: the bitmap itself is an array of unsigned int, which
  eliminates alignment padding between the n_bit field and the bitmap.

* more unit tests.