Add a shortcut to speed up concrete evaluation of pmod.
Add a shortcut to speed up concrete evaluation of pmod.

Used with a modulus of degree `d`, the low `d` bits of
the input can be copied directly to the output unchanged.
This lets us skip the first `d` iterations of the main loop.

GaloisInc/cryptol,GaloisInc/cryptol,GaloisInc/cryptol