Add datastructure class RandomGrabHashSet


- I had written this for WOT originally. It is very generic so toad asked me to add it to Fred.
- Notice that it is not duplication of RandomGrabArray: RandomGrabArray is very tightly wired in to non-generic Fred code. RandomGrabHashSet is a naked datastructure which doesn't depend on Fred at all.

I would even prefer not to use random values at all. Pick a couple of values that have some relevance for the algorithm being tested (i.e. upper/lower bounds, 0, any kind of special cases) and test those.