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.

Well a hashset usually is backed by an array, and is a very common datastructure, so I though people might know what the "mArray" of an hashset is.
What would you like instead - "[m]HashArray" ? "[m]HashSet" ? "[m]Data" ?