Python API using Python CFFI

[rkaminsk]

• set python version via definition
• fix remaining pylint messages
• add install target for python library
• find a nice way to handle the path in embedded pyclingo
  • there should be a path to be able to run clingo without installing
  • otherwise, it is probably best to not set a path
• handle visibility of declarations in embedded pyclingo
  • I did not find a way to change visibility without manually editing the files.
  • Currently, the cmake files contain a crude workaround to make it link on Windows.
  • Embedding does also not work nicely on Windows because some paths are missing.
    They can be set manually and and at least the python module works nicely.
  • This might need further fine-tuning (or maybe disable embedding on Windows).
• improve documentation
  • Python documentation
  • C documentation
• create pip package
  • try to reuse the code from the pip branch to do this
• Create C++ bindings for new AST API.
  • implement bindings
  • add more tests
• Implement unpooling (or maybe later).