Easy constructor for additive/multiplicative groups.

This let's us easily grab the additive/multiplicative groups (and
monoids, semigroups) using, eg, Group.additive[Long]. It is nice because
we do not need to worry about the structure it comes from. For example,
Group.additive[Vector[Long]] works and you don't need to worry about
Module[Vector[Long], Long].
give KleeneDemo a better name
Commit mandelbrot set demo
clean up WellPrng and fix some bugs
Merge remote-tracking branch 'tixxit/topic/easy-group' into 2.10.0