Enforce class members are initialized

```py
class A:
a: int
def __init__(self, a: int):
print("LOOL!")

print(A(1).a)
```

I would expect an error:
`a: int # no default value and not set in initializer`

The issue names no implementation file or test, so begin by locating the checker logic for annotated class members and constructor analysis. Use the A example as the reproducer; done means the checker reports that a has no default value and is not assigned during initialization.

Written by the indexing model from the issue text.