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.

Interfaces—per definition—do not add any runtime cost. There are, indeed, some additional functions that need to be implemented, especially for ```java.util.Collection``` of which ```Set``` is a subinterface. Most are straightforward, the few harder ones are optional (and may hence throw ```UnsupportedOperationException``` if nobody feels inclined to code them). The only one that is requires actual coding is ```iterator()```, but even that one is relatively straightforward (just make a wrapper around ```data.iterator()``` that also updates ```index``` on ```add()```/```remove()```).