Add Well512a and Well1024a PRNG implementations

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