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.

Comienza con el issue de dependencias #15906 y la serie posterior del pull request #15287. Lee las rutas de inferencia actuales y la lógica de apply_poly() en checkexpr.py; después, revisa applytype.py. Se considera terminado cuando la inferencia polimórfica se use durante la unificación de subtipado y como mecanismo principal para la inferencia del tipo de retorno y de ambos pases de inferencia de argumentos.