Use new style CFFI callbacks in CFFI backend

[thedrow]

See http://cffi.readthedocs.io/en/latest/using.html#extern-python-new-style-callbacks

[rtobar]

I have an initial set of changes in a local repo that implement this, but not in a publishable form yet. Performance-wise this doesn't bring much benefit at the moment though compared with other parts of the code, and it of course imposes the requirement of a compiler during installation (which could be checked in setup.py).

[isagalaev]

Looks like this is fixed by #56.