k_mul: Rearranged computation for better cache use. Ignored overflow

k_mul:  Rearranged computation for better cache use.  Ignored overflow
(it's possible, but should be harmless -- this requires more thought,
and allocating enough space in advance to prevent it requires exactly
as much thought, to know exactly how much that is -- the end result
certainly fits in the allocated space -- hmm, but that's really all
the thought it needs!  borrows/carries out of the high digits really
are harmless).