Consider using pybind11 in C-backend

[ferrine]

The idea is to simplify our c++ backend using
https://github.com/tbenthompson/cppimport
and
https://github.com/pybind/pybind11

[brandonwillard]

See #376 (comment) first. If we could use Cython for the C-backend, we would solve numerous issues and make it very easy to add new and improved C implementations. That's currently the highest priority for C-backend refactoring.