cleanup pass over clingo.hh (#209)
cleanup pass over clingo.hh (#209)

This removes quite some clang-tidy warnings but there are still a lot of
them left. The remaining ones are all harmless and can be ignored.
Unfortunately, I did not find a directive to disable certain warnings
for the whole file without sprinkling NOLINT comments all over the
place. Closes #209.

potassco/clingo,potassco/clingo,potassco/clingo,potassco/clingo