Modular arithmetic for Mersenne numbers

[make-github-pseudonymous-again]

Let r be the radix of representation, then computing x % r^k-1 can be reduced using the following identity (with R = r^k):

x = x % R + x // R (mod R-1)

See https://en.wikipedia.org/wiki/Lucas%E2%80%93Lehmer_primality_test#Time_complexity

[make-github-pseudonymous-again]

This also works for numbers of the form R-c:

Let x = hi * R + lo, then x = hi * (R-c) + c * hi + lo = c * hi + lo (mod R-c).