Change Subst to store a single mapping for vars

Change Subst to store a single mapping for vars

The previous implementation stored a Vector of values mapped, and adding
new mappings shadowed previous ones. I did it like this because the
implementation in the paper used association lists, which has this
shadowing effect. But, the implementation in Clojure simply uses a
dictionary like I'm using, and after doing these examples without seeing
shadowing, I'm convinced that it has the same behavior.

During this change, I also ended up changing a bunch of places with
cloning to pass references instead, which should cut down on reference
counting bookkeeping time.