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.

Checked the compatibility to the Set interface - it is mostly given. The primary difference is add() which throws instead of returning. And that difference is justified: The class is used by WOT for unit tests, and in those tests adding somthing twice to the same set would indicate an error. So it should throw then. This is also a good reason of not making this class implement Set.
I've documented this in the JavaDoc of add() at https://github.com/freenet/plugin-WoT-staging/commit/23d4eec4e40b5f02ca2748a1ee6c88bc1b37ac56
 Case closed :) Thank you