Faster bit map lookup

[benhj]

E.g. this allocator code:

int64_t bmp_alloc(uint64_t *bmp, int64_t slots){
  // search for the first 0 bit, set it to 1,
  // then return the slot
  // return -1 if nothing found
  slots /= 64; // checking 64 slots at a time
  for (int64_t loop = 0; loop < slots; loop++){
    if (*bmp == 0xFFFFFFFFFFFFFFFFLL){
      // this area is full, go to the next area
      bmp++;
      continue;
    }
    // we're guaranteed at least one bit is 0
    int pos = ffsll(~*bmp) - 1;
    *bmp |= 1LL << pos; // set the bit at pos
    return loop * 64 + pos;
  }
  return -1;
}

From http://syntheti.cc/article/kongs-garbage-collector/

Could be similarly used to do far quicker bitmap lookup. Presently, I'm checking bit by bit

[benhj]

Done. (Note: if not in master, in debuggingSeek dev branch)