This is not a pure refactoring, but almost. Right now we are in a weirdsituation where we have two inconsistencies:* `__set__()` is handled in `checker.py` while `__get__()` is handled in`checkmember.py`* rules for when to use binder are slightly different betweendescriptors and settable properties.This PR fixes these two things. As a nice bonus we should get freesupport for unions in `__set__()`.

Fixes#18988This should be a minimal change to restore backwards compatibility foran edge case with forward references.

Fixes#18967Fixes#16494Fixes#15793Fixes#12949As you can see from updated test cases, it is kind of gray area, sowhether we go this way will depend on the `mypy_primer` results (andalso potentially on Dropbox internal code bases, where the above issuemay cause problems).

Fixes#18856. This should be done by `TypeAnalyzer.anal_array` but isnot - semanal only invokes its own wrapper around `anal_type`---------Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>

…1.16

The feature was introduced in#18727.

Also discuss `mypyc_attr(native_class=<...>)`.