Optimize numeric multiplication for one and two base-NBASE digit multiplicands.

Hello hackers,

Attached patch introduces an optimization of mul_var() in numeric.c, targeting cases where the multiplicands consist of
onlyone or two base-NBASE digits. Such small multiplicands can fit into an int64 and thus be computed directly,
resultingin a significant performance improvement, between 26% - 34% benchmarked on Intel Core i9-14900K.
 

This optimization is similar to commit d1b307eef2, that also targeted one and two base-NBASE digit operands, but
optimizeddiv_var().
 

Regards,
Joel