Use new type inference as a primary mechanism

[ilevkivskyi]

Depends on #15906. This is a follow-up for series started by #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.