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.

Can that actually be done efficiently while preserving the O(1) random grab and not adding a lot of extra complexity? IIRC Set is pretty heavyweight. I agree it'd be useful to be able to pass them as Set's, but it may not be worth the code and runtime cost?