Improving ImplementInterface code fix

[psfinaki]

Changes are small (review without whitespace) and include:

• move to ctasks
• removing some duplicated async calls
• tests
• half-assed fix of "Implement Interface" defines multiple methods for nested interfaces #5813: as in, the code fix just doesn't trigger in that case now - generating correct fixed code should be doable but hard and not worth my effort given it's a rare case