Formalize Python package for gccjit

Fix problematic Comparison enum definitions which were not using "c_api."
in the assignment.

Move cython files under package sub-folder "gccjit" and include a Python
__init__.py file which imports all the relevant API. This will allow for
more API modification in pure Python and placement of additional
Python modules within the package.

Move test.py into "tests" sub-folder and hook into optional usage of
setuptools when available. This allows running of tests from setup.py
with the optional ability to specify single tests:
$ python3 setup.py test --test-suite tests.test.JitTests.test_square