Isolate CanonicalSet as a subclass of WeakHashSet

WeakHashSet is currently pulling double duty:

• It functions as a normal set that happens to store its entries using WeakReferences
• it has additional methods (ie canonicalize) in order to function in a manner similar to String.intern

We should seperate these two concerns - primarily so what is going on can be understood.

• WeakHashSet - remains as set of weak references, methods providing additional functionality will be deprecated
• CanonicalSet cerated as an extension to WeakHashSet and proivdes a unique method

This bug report is done when we have updated the code and provided user docs on the result.

Sample use:

class FooFactory {
    public Foo create(String definition) {
          Foo created = new Foo(definition);
          return (Foo) canionicalSet.unique(created);
    }
}