[native driver] Maintainability improvements

[juanjux]

• Removed old cruft.
• AstImprover: Use light classes for tokens and positions instead of the native tokenizer tuples.
• AstImprover: Remove dependency from the "six" module since it'll only be executed under Python 3.
• AstImprover: Add static typing to the module (it was the only one missing it because it was inherited from pydetector).
• AstImprover: Fixed a couple conversion/reinterpretation bugs found thanks to the static typing.