Use new type inference as a primary mechanism

Depends on https://github.com/python/mypy/issues/15906. This is a follow-up for series started by https://github.com/python/mypy/pull/15287

Currently new (polymorphic) inference is used as a fallback if the traditional algorithm fails (one way or another). At some point we should switch to the new inference scheme entirely. This would include:

• Use polymorphic inference during unification in subtyping.
• Use polymorphic inference as primary mechanism for all inference passes (return type, arguments first pass, arguments second pass).
• Move apply_poly() logic from checkexpr.py to applytype.py.

Inizia con la dependency issue #15906 e con la serie successiva della pull request #15287. Leggi i percorsi di inferenza correnti e la logica di apply_poly() in checkexpr.py, quindi esamina applytype.py. Il lavoro è completo quando l’inferenza polimorfica viene usata durante l’unificazione del sottotipaggio e come meccanismo principale per l’inferenza del tipo restituito e di entrambi i passaggi di inferenza degli argomenti.