Public interfaces accept NSFastEnumeration instead of RXTraversal.

RXTraversalArray has been renamed RXEnumerationArray as part of this, and the RXTraversal protocol covers the RXTraversal class interface more directly. Thus, if you have an RXTraversal instance, you can expect to be able to call -isExhausted and -consume: on it.
The RXTraversal class is now private.