Avoid naming collision during synthesis

[datvo06]

This PR:

1. Add a AST-walking procedure to rename all colliding functions within the synthesized module.
2. Also walk the body/FunctionDef recursively for renaming.
3. Corresponding tests.

closes #542

[datvo06]

One subtlety is that the rename happens only inside the copied module during mypy type checking, so when we return the module after type checked, it is unmodified.