Function pointer with function pointer arg needs diagnostic

[dtolnay]

// src/lib.rs

#[cxx::bridge]
mod ffi {
    unsafe extern "C++" {
        include!("example/include/header.h");

        fn f(x: fn(y: fn()));
    }
}

// include/header.h

#pragma once
#include "rust/cxx.h"

void f(rust::Fn<void(rust::Fn<void()> y)> x);

This currently emits a warning but otherwise compiles (and probably does the wrong thing). It needs to error instead because we don't yet fully support function pointers getting passed in the C++-to-Rust direction -- tracked in #52.

warning: `extern` fn uses type `fn()`, which is not FFI-safe
 --> src/lib.rs:6:23
  |
6 |         fn f(x: fn(y: fn()));
  |                       ^^^^ not FFI-safe
  |
  = note: `#[warn(improper_ctypes_definitions)]` on by default
  = help: consider using an `extern fn(...) -> ...` function pointer instead
  = note: this function pointer has Rust-specific calling convention