Use `FnOnce` in API where applicable

Functions like https://docs.rs/libpulse-binding/2.26.0/libpulse_binding/context/introspect/struct.Introspector.html#method.get_server_info take `FnMut`. The function shouldn't be called more than once, right? Then `FnOnce` is more general and appropriate.

I guess this would apply to a lot of callbacks...