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.

Ok, I was thinking of ordered set or something, which has all sorts of stuff around subsets/slices/etc...