Consider Montgummary multiplication for Polard Rho

[oscardssmith]

idea comes from https://en.algorithmica.org/hpc/algorithms/factorization/. Lenstra will still be better for numbers larger than Int64, but getting extra speed is always nice.

[ndcroos]

Hi, I'd like to work on this issue.

[oscardssmith]

sounds great! I started implementing montgommary multiplication in JuliaMath/IntegerMathUtils.jl#1 but never got it fully working. That's probably the first step.