hv.c/handy.h - move logic from hv.c to handy.h

These mathematical/RNG functions are handy for various
internal uses, so expose them in a way they can be reused.

These are Marsaglia RNG "permutation" XORSHIFT triplets, from the famous
article he wrote on fast random number generation. By modern standards
they are only mediocre as far as RNGs go, but they still aren't bad. But
they are very fast, and have a few useful properties which can be handy
in places where we want a cheap source of pseudo random bits, or where
we want to get a random(ish) sequence of positive integers.

This will be used in a follow up patch.

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