Rename bn_umul_hilo() to bn_mulw().

Rename bn_umul_hilo() to bn_mulw().

This keeps the naming consistent with the other bignum primitives that have
been recently introduced. Also, use 1/0 intead of h/l (e.g. a1 instead of
ah), as this keeps consistency with other primitives and allows for naming
that works with double word, triple word and quadruple word inputs/outputs.

Discussed with tb@