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.

The code was originally written for WOT, and this is the style convention there. Feel free to change it for this class when merging.
(Changing WOT to not use this is not an option as it is done like that EVERYWHERE and this would also require messing with db4o to make it rename its internal binding of the variables).