Use CFFI instead of ctypes

CFFI outperforms ctypes, even on CPython.
It will also make arrayfire-python the first library that enables GPU access from PyPy.

Start by locating the existing Python ctypes bindings and tracing how they expose ArrayFire GPU access. Compare the current binding surface with the CFFI approach, then verify that GPU access remains available on PyPy and that the migration meets the stated performance goal.

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