Revert opam solver dependency changes

- Reverts the change to the monolithic `clingo` package in favor of the
  previous split between `clasp` and `gringo` since `opam` works with
  the latter but not (for some reason) with the former.

- Adds explicit non-support for Python in `gringo`. This is not necessary
  for opam.

- Forces usage of the `std::to_string` functions in the C++ standard
  library instead of the incomplete alternative implementations inside
  of the `gringo` sources.