bitmap: various clean-ups

bitmap: various clean-ups

a) use memcmp() to compare bitmaps efficiently

b) use UINT64_C() macro instead of ULL suffixes to get right suffix for
   uint64_t constants

c) add a few assert()s

d) when comparing integers with 0 we generally try to make this explicit
   with "!= 0".

e) remove redundant bitmap_isset() if check, as we don't have it in
   bitmap_isset() either.

f) It should be fine to invoke bitmap_unset() on a NULL bitmap