Enable functional-like behavior with 'persistent' interfaces

[sdboyer]

functional style dictates that all state mutations should return a new instance of the underlying datastructure (which can be accomplished by mapping into a tree somewhere). the mutable interfaces should be changed to accommodate this pattern (though not necessitate it) by adding return values that are of the interface mutator type.

[sdboyer]

on further reflection, it's really not possible to do this in a way that's fully consistent with the existing Mutable interfaces; adding return values would introduce an inconsistency that should not be inflicted on client libs, as the return value from an imperative-style mutable would be the same pointer as the original method receiver, but the return value from a functional-style mutable would be a different pointer.

consequently, a Persistent set of interfaces is a better approach; these would add the return values on methods of the same name as what's already on Mutable. thus, it'd be impossible to implement both the persistent and mutable interfaces at the same time.