k_mul(): The fix for (ah+al)*(bh+bl) spilling 1 bit beyond the allocated

k_mul():  The fix for (ah+al)*(bh+bl) spilling 1 bit beyond the allocated
space is no longer needed, so removed the code.  It was only possible when
a degenerate (ah->ob_size == 0) split happened, but after that fix went
in I added k_lopsided_mul(), which saves the body of k_mul() from seeing
a degenerate split.  So this removes code, and adds a honking long comment
block explaining why spilling out of bounds isn't possible anymore.  Note:
ff we end up spilling out of bounds anyway <wink>, an assert in v_iadd()
is certain to trigger.