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.

There's nothing wrong with using randomly-generated values in tests, especially if they are repeatable with a known seed. It can be quite handy to catch issues you didn't originally think of.