Introduced helper functions v_iadd and v_isub, for in-place digit-vector

Introduced helper functions v_iadd and v_isub, for in-place digit-vector
addition and subtraction.  Reworked the tail end of k_mul() to use them.
This saves oodles of one-shot longobject allocations (this is a triply-
recursive routine, so saving one allocation in the body saves 3**n
allocations at depth n; we actually save 2 allocations in the body).