Refactor BN_mul().

Refactor BN_mul().

This splits BN_mul() into two parts, one of which is a separate bn_mul()
function. This makes the code more readable and managable, while also
providing a better entry point for assembly optimisation. A separate
bn_mul() is provided for the BN_RECURSION implementation, to reduce
complexity.

This also enables bn_mul_comba4() for four word long bignums - this was
disabled for unknown reasons.

ok tb@