`Operation` should unify with `Callable`

The following should work:

```
from typing import Callable

from effectful.ops.types import NotHandled, Operation


@Operation.define
def f(x: int) -> int:
    raise NotHandled


@Operation.define
def g[S, T](x: Callable[[S], T]) -> T:
    raise NotHandled


t = g(f)
```