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.

@bertm : I am strongly for using existing interface infrastructure and therefore also considered using the Set interface. But it would add a LOT of functions which the code which uses this class does not need, and I didn't want to write a total bloat of a class. If someone needs the Set interface, it can easily be added as the class probably already uses function names which are consistent with it. I will check the functions again to make sure that they would conform to the Set interface.