Numba integration

The basic idea is that libgumath contains functions that allow inserting gufuncs and kernels into the lookup table.

Ideally, Numba would jit-compile specialized kernels and call the insertion function (must be on the C level for safety).

The function is then automatically available to be called on the Python level via the gumath Python module.

Obstacles

• If the datashape (ndt_t) signatures are given on the Python level (which is probably the only sane option), the jit-compiled kernel needs to be type-checked against the ndt_t type.