Fixes #433.
This does not break any code, but allows more code to compile, as stated in that issue and in the Fn docs:

Since both FnMut and FnOnce are supertraits of Fn, any instance of Fn can be used as a parameter where a FnMut or FnOnce is expected.

Use Fn as a bound when you want to accept a parameter of function-like type and need to call it repeatedly and without mutating state (e.g., when calling it concurrently). If you do not need such strict requirements, use FnMut or FnOnce as bounds.