More principled solution for universal quantificatoin of non-Callable types

[ilevkivskyi]

Currently we support a type like forall T . Foo[[x: T], T] by allowing to capture a generic callable with a ParamSpec. This is arguably hacky, and may fail in unpredictable ways. Also such types are not user-definable. At some point in distant future we may want to add a support for defining such types.