Switch to pybind11

[pastewka]

Should we switch the C code to C++ and pybind11? My guess is this would lead to a C++ code that is half the size of what we have currently, but we would have a dependency on pybind11 (which is actually just a bunch of C++ headers).

[jameskermode]

I don't have any objection to this. We already have some C++ code so there are no further portability concerns beyond the pybind11 dependency.

[pastewka]

Okay, I'll see if can make room to do this. It would greatly simplify many aspects of the interface (and make it more robust also with respect to type confusion issues, i.e. #95)

[pastewka]

We should probably switch the build system first, see #116

[pastewka]

Will take care of this