k_mul() and long_mul(): I'm confident that the Karatsuba algorithm is

k_mul() and long_mul():  I'm confident that the Karatsuba algorithm is
correct now, so added some final comments, did some cleanup, and enabled
it for all long-int multiplies.  The KARAT envar no longer matters,
although I left some #if 0'ed code in there for my own use (temporary).
k_mul() is still much slower than x_mul() if the inputs have very
differenent sizes, and that still needs to be addressed.