Proposal: Callable variant types

Summary I propose adding different variants of Callable to typing or other relevant parts of the stdlib, which use the same syntax as Callable but allow differentiating between different kinds of callables which may have different semantics. Motivation Callable is relatively broad, which means type checkers and authors have to make assumptions which makes it harder to write correct types for many cases where Callable is currently used. The exact semantics of Callable have also grown over time, ...