Ssort

Contains a massive refactoring of requirement and binding extraction by
jgberry, which fixes a number of bugs. In particular:

- Fixes bindings not being discovered in return statements.
- Fixes bindings not being discovered in exception handler type expressions.
- Fixes dict not resolving requirements from its keys.
- Fixes augmented assignment not resolving requirements from its target.
- Fixes annotated assignment not resolving requirements from its annotations.
- Fixes functions and lambdas not resolving requirements from their arguments and annotations.
- Fixes with statement not resolving requirements from its optional_vars.

Fixes for statement not resolving requirements from its target.

In addition, enables walrus tests for Python 3.8 and introduces a new
API for hooking in to error reporting.

Breaking changes:
- Moves inner classes to top of class body instead of sorting them somewhere in the middle.

Other changes:
- Relaxes topological sorting of class members to only apply to private members.

Bugfix release containing backport of 55

Adds support for walrus bindings in format strings. Thank you jgberry.

Fix stripping of trailing comments from files