Allow Callable type hint before function definition

Feature

Allow

f: Callable[[int], int]


def f(n):
    return 42

as an alternative to

def f(n: int) -> int:
    return 42

Pitch
Currently, the proposed syntax produces a Name "f" already defined ... [no-redef] error, unlike similar code for non-function values:

# OK
x: int
x = 3

For some complicated function signatures, it would be clearer to provide a single Callable hint than to have argument types
mixed in with the arguments themselves and the return type added after the parameter list. For that matter, the two styles above need not be mutually exclusive:

f: Callable[[int], int]
def f(n: int) -> int:
    return 42

mypy should report an error only if the types are identical. (Or perhaps compatible; I don't know if there would be use-cases
for the initial type to be a supertype of the one implied by the function annotations.)

This would be consistent with the def statement being a kind of assignment, as the following is currently allowed:

x: int
x = 3

as well as eliminate the asymmetry between function parameters and "ordinary" names.

The issue names no file, test, or entry point. Start by locating mypy's handling of annotated assignments and function definitions, then compare how it reports duplicate names. Done means the Callable declaration before def is accepted, compatible annotations are handled consistently, and conflicting types still produce an error.