Remove old support lib.
Fixed PPrint
Tried using auto-induction in a couple more places, unsuccesfully
preparing for some more advanced examples with local state etc.
Removed useless stuff from preservation proof
Adding let to stlc (exercise); hit incomplete pattern matching bug

... I think :)
Fixed context_invariance with lets
Done with let, it's an interesting exercise, but quite hard
Solution for adding pairs stlc exercise

Simpler than adding let, but also less interesting
das
Extended support to support ocaml gen
update VS solution version