mib: convert C module for CPython to CFFI

This will allow to use Pypy and to convert to Python3 more
easily. CFFI seems safe enough for our use.
setup: correctly depends on CFFI

We need to ensure that CFFI is present for setup. If not, we need to
not add the verifier as an extension. setup.py will first install CFFI
and reexecute itself and then, the extension will be present as a
dependency.
mib: compatibility with Python 2.6.

In Python2.6, `format()` needs indexed placeholders.