Remove the last bn_wexpand()s that made us break constness.  Of
course, that means we need to handle the cases where the two arrays to
bn_mul_recursive() and bn_mul_part_recursive() differ in size.

I haven't yet changed the comments that describe bn_mul_recursive()
and bn_mul_part_recursive().  I want this to be tested by more people
before I consider this change final.  Please test away!