Fixes python#18024
Fixes python#18706
Fixes python#17734
Fixes python#15097
Fixes python#14814
Fixes python#14806
Fixes python#14259
Fixes python#13041
Fixes python#11993
Fixes python#9585
Fixes python#9266
Fixes python#9202
Fixes python#5481

This is a fourth "major" PR toward
python#7724. This is one is
watershed/crux of the whole series (but to set correct expectations,
there are almost a dozen smaller follow-up/clean-up PRs in the
pipeline).

The core of the idea is to set current type-checker as part of the
global state. There are however some details:
* There are cases where we call `is_subtype()` before type-checking. For
now, I fall back to old logic in this cases. In follow up PRs we may
switch to using type-checker instances before type checking phase (this
requires some care).
* This increases typeops import cycle by a few modules, but
unfortunately this is inevitable.
* This PR increases potential for infinite recursion in protocols. To
mitigate I add: one legitimate fix for `__call__`, and one temporary
hack for `freshen_all_functions_type_vars` (to reduce performance
impact).
* Finally I change semantics for method access on class objects to match
the one in old `find_member()`. Now we will expand type by instance, so
we have something like this:
  ```python
  class B(Generic[T]):
      def foo(self, x: T) -> T: ...
  class C(B[str]): ...
  reveal_type(C.foo)  # def (self: B[str], x: str) -> str
  ```
FWIW, I am not even 100% sure this is correct, it seems to me we _may_
keep the method generic. But in any case what we do currently is
definitely wrong (we infer a _non-generic_ `def (x: T) -> T`).

---------

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>