an `AttributeOf` form

like `keyof` in ts:
```py
def f[T](t: T, a: AttributeOf[T]):

f(1, "real") # ok
f(1, "fake") # error
```

Use the issue's generic function example as the starting point and inspect the type-checker entry points for existing typing constructs. Define what AttributeOf[T] should accept, then verify that an existing attribute such as "real" is accepted while "fake" is rejected for the shown call.

Written by the indexing model from the issue text.