Add cloneUnderlying method to Namespace

This adds a method, cloneUnderlying, to Namespace that returns a copy of
the underlying mutable.HashSet. This is useful for constructing a
Namespace that you would like to manipulate manually without using
Namespace's methods to generate temporaries.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>