[Alt abd] native Z3 integration with ocaml bindings

This re-implements the glue with Z3 using the OCaml API.

The Z3 libraries are now included in the repository, in [libext/], and made into a caml module ([libext/META.z3]) so that everyone can compile coreStar without having to get Z3's latest ml-ng branch.

Side-effects:
- We'll have to manually keep in sync with Z3, but hopefully at some point the OCaml bindings will make it into an official release and we'll drop [libext/].
- The Z3 files are under a different software license (see [libext/LICENSE.txt]).
- coreStar is now slightly faster :)