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.

依存関係の issue #15906 と、それに続く pull request #15287 のシリーズから始めてください。現在の推論パスと checkexpr.py の apply_poly() のロジックを読み、その後 applytype.py を確認してください。subtyping の単一化中、および戻り値の型と両方の引数推論パスにおける主要なメカニズムとして、多相推論が使用されれば完了です。