Decouple pwnlib and pybind11 from CRAX

[aesophor]

Currently CRAX relies on pwnlib (and thus on pybind11) for ELF parsing, so some changes are made to S2E's makefiles. Maybe we could decouple pwnlib (and thus pybind11) from CRAXplusplus, so that this repo doesn't have to hold the entire s2e.

[aesophor]

Maybe reuse libvmi from s2e.