Support the CUDA Array Interface

This commit adds support for version 3 of the CUDA Array Interface
(https://numba.readthedocs.io/en/latest/cuda/cuda_array_interface.html):

- GPU Arrays export the interface so that they can be passed to other
  supporting libraries.
- Function calls check for the presence of the interface and use the
  data pointer from it if found, so other libraries' arrays can be
  passed to PyCUDA-compiled kernels.

Documentation, tests, and examples for the interface going in both
directions are added.