Support a hierarchical namespace

[klinvill]

Currently, everything in Ravencheck is encoded into a flat namespace, except for enum constructors. This leads to a clash when functions for different sorts are given the same name. For example, both sets and lists often are defined with a membership function named mem.

It would be nice to have ravencheck encode names using the namespace, so that I could use import set and list, that both define mem, and use set::mem and list::mem in the same module.