removing AutoProver2 (AutoProver3 subsumes it).
A few big changes.

+ Changed ExprI to use exprD'_weaken instead of having
  separate lemmas for UVars and Vars.
  - This is made possible by a better implementation of some
    lemmas.
+ Even more complete proof of AutoProver2
  - The only thing left is to reason about instantiations.
- Commented out the demos for now, they need a few more tweaks
+ Some work on standardizing Lemma
+ Some cleanup work on instantation.