Fully implement ConstDict and improve API

[braxtonmckee]

ConstDict is only partially implemented. It was originally intended to be a tree data structure but currently only the single-leaf-list form is implemented. Its also not fully compiled. Finally, the type coercion operators are not very well defined - for instance when you subtract anything from it, it tries to convert it to a TupleOf. What should we do if it fails - should we be raising NotImplemented and allowing reverse-operator behavior? Or should we simply be trying to iterate whatever we've been given and subtracting it.