rdar://107584830

Currently, when iterating over the bits of a WTF::Bitmap, we manually iterate bit by bit. On x86_64 and ARM64 however, we should generally have access to bit-counting instructions in hardware. This means instead of doing one iteration for every 1 or 0 bit, we still do one iteration for every 1 bit, but skip up to a word of 0 bits at a time. This can massively reduce iteration time especially for sparse bitmaps.