derive support for C-like fn pointers and core::ptr::NonNull

[Crzyrndm]

When generating a Rust binding to a C struct containing function pointers, bindgen outputs type definitions that look roughly like the following

pub type c_library_callback_t = ::core::option::Option<
    unsafe extern "C" fn(
        param1: Param1Type,
    ),
>;

#[repr(C)]
#[derive(Debug)]
pub struct c_library_api {
    pub callback: c_library_callback_t 
}

with the bindgen ParseCallbacks::add_derives interface, I can add defmt::Format to all of the bindings without issue, however I currently need to manually make exceptions for any type that includes C function pointers and any struct including those structs.

The std Debug impl is just printing the function pointer. Don't see any reason why defmt shouldn't do exactly the same

On a strongly related topic, core::ptr::NonNull is a fairly common need when wrapping a generated C-API (at least, from what I've written wrappers for) and doesn't currently have defmt support