simple_rng/mash.rs tests

[joshfactorial]

Not 100% sure how to test this or what degree of testing it needs. I changed a few things in translating from javascript, but it produces the same results with the same input. Adding some further tests beyond what is present and offering recommendations for any code improvements the testing reveals would be great. If no further tests are needed, adding /// documentation, as in simple_rng/lib.rs would also be useful