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.

Fixed in  commit 224e7defab117717850bb584e1937764e8342584. The code has been moved to the WOT repository already, so the commit is there.