Add optional C extension wrapper for Python JSON parsing

Add optional C extension wrapper for Python JSON parsing

The pure Python in-tree JSON parser is *much* slower than the
in-tree C JSON parser. A local test parsing a 100Mb JSON file
showed the Python version taking 270 seconds. With the C wrapper,
it took under 4 seconds.

The C extension will be used automatically if it can be built. If
the extension fails to build, a warning is displayed and the build
is restarted without the extension.

The Serializer class is replaced with Python's built-in
JSON library since the ability to process chunked data is not
needed in that case.

The extension should work with both Python 2.7 and Python 3.3+.

Signed-off-by: Terry Wilson <twilson@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>