prototype: use explicit float bitmask

NOTE: return value is garbled because I didn't split up the commits exactly
right. The next commit should return some non-garbage (the dot product).
prototype: try sse4, sse3 and sse2 dot product

It looks like they produce the same results, the sse4 and sse3 versions are
verifiable on sight, the sse2 version might require a bit of pen and paper.
readme, notes: update