Consider adding a ChainMap-like state class

[brandonwillard]

With a ChainMap-like state class, we could constrain the amount of state (e.g. dict) cloning that kanren does, which should provide a big performance improvement when states are large Mappings (e.g. the kind produced by our graph relations/goals).

N.B. A lot of the state cloning is implicitly done within unify, so this might be more appropriate for https://github.com/pythological/unification.