[red-knot] Correct modeling of dunder calls (#16368)
## Summary
Model dunder-calls correctly (and in one single place), by implementing
this behavior (using `__getitem__` as an example).
```py
def getitem_desugared(obj: object, key: object) -> object:
getitem_callable = find_in_mro(type(obj), "__getitem__")
if hasattr(getitem_callable, "__get__"):
getitem_callable = getitem_callable.__get__(obj, type(obj))
return getitem_callable(key)
```
See the new `calls/dunder.md` test suite for more information. The new
behavior also needs much fewer lines of code (the diff is positive due
to new tests).
## Test Plan
New tests; fix TODOs in existing tests.
