Faster quaternion multiplications.

Use the known factorization trick to speed-up quaternion multiplication.
Now we need only 9 floating-point multiplications, instead of 16 (but
at the cost of extra additions and subtractions).

Callgrind shows that the function now takes 299 instructions instead of
318 instructions, which is not a big win. However I assume the speed-up
has a better effect for mobile CPU, where multiplications are more
expensive.

Reviewed-by: Rhys Weatherley