libspotgen

It would be nice to have all the LTL generators of genltl available in Python. This means we should move those functions outside of bin/genltl.cc; but I would also prefer to not define them in libspot.so, because that is really unlikely to be used in most tools built using Spot, and if we continue adding new hard-coded lists of formulas, it could become quite big. So I'd suggest to create a libspotgen.so library, with Python bindings in package spot.gen, and also its own Debian packages.

@max just suggested we might want a genaut binary as well, and I would put the functions constructing these automata in libspotgen.so as well.