Rewrite generator to use CST (and more)

Rewrote most of the generator to use LibCST. This provides a concrete syntax tree parser.

Advantages:

• Formatting is no longer messed up due to removal of nodes
• Generator much more consistent (order of decorators/comments used to matter, not anymore)
• Resolves #2 (closed)
• Resolves #5 (closed)
• Resolves #7 (closed)
• New features can be added relatively easy (compared to the old version)
• There is better documentation in the code (part of #6 (closed))
• There are tests (resolves #8 (closed))

Disadvantages:

• Longer run time

Given the severity of the changes, I hope all of you can take a look at this MR and try it out 🙂