Reorganize dirs and switch to autotools

Reorganize the directories as discussed with @hellais. I've
not moved libneubot.py and libneubot.json, tho, because it's
not clear where to move them.

Since the directories reorganization breaks the old Makefile,
this is a good moment to switch to autotools as well.

While there, remove a couple of things (gen_cxx, neubot.hh,
libneubot_python/) that were already deprecated, to avoid
waisting brain cycles to decide where to put them.

All this work completes step one of the transition from libneubot
to libight. The next step (of which I will take care) consists of
renaming all the symbols to use the Ight (or ight) prefix instead
of the Neubot (or neubot) prefix.