Add Well (512a, 1024a, 19937a, 19937c, 44497a and 44497b) PRNG implementations, raise the treshold in GeneratorTest from 0.3% to 0.35%

* Adds implementations of `Well512a` and `Well1024a` PRNGs.

* Suggests deprecating the current `Well512` implementation as it generates the same PRNG values (is functionally equivalent) and is IMHO inferior in terms of readability, abstraction and similarity to `Well512a.c` and furthermore inconsistent and redundant with the new `Well1024a` and `Well512a`.

* Factors out `MersenneTwister32/64` utility functions into spire.random.mutable.Util.