swig->pybind11 compat wrapper

There should be a wrapper so people can continue using the swig interface with the implementation switched over to pybind11.

The tests in https://github.com/google/s2geometry/blob/master/src/python/s2geometry_test.py should pass.

This means the `%unignore`d functions in https://github.com/google/s2geometry/blob/master/src/python/s2_common.i need to be in the pybind11 bindings.

@deustis