Adds -nextObject to RXEnumerator.
Filter tidying.
RXEnumerator (the class) declares its conformance to NSCopying.
RXFilter’s enumerator copies its stop state.
RXMap uses enumerators.
Tidying.
Interval tests don’t rely on -currentObject.
Enumerators only provide -nextObject.

This also removes the stop parameters.

Maps and folds can no longer produce nil, but the semantics of the API
are much more explicit.

Producing nil/early termination via some other sigil value will be
reintroduced via a -nextObjectOrSigil: (or similar) method.
Fixes RXLinearSearch.