Implement function pointer passing from Rust to C++

[dtolnay]

For example:

mod ffi {
    extern "C" {
        fn c_take_callback(callback: fn(String) -> usize);
    }
}

This corresponds to an argument of type rust::Fn<​> on the C++ side, with an operator() to call the Rust function pointer.

void c_take_callback(rust::Fn<size_t(rust::String)> callback) {
  callback("...");
}

---

The generated code is currently as follows; we can make this better in subsequent PRs.

void cxxbridge02$c_take_callback(::rust::Fn<size_t(::rust::String)> callback) noexcept {
  void (*c_take_callback$)(::rust::Fn<size_t(::rust::String)>) = c_take_callback;
  c_take_callback$(callback);
}

size_t cxxbridge02$c_take_callback$callback$1(::rust::String *_0, void *) noexcept;

size_t cxxbridge02$c_take_callback$callback$0(::rust::String _0, void *extern$) noexcept {
  return cxxbridge02$c_take_callback$callback$1(&_0, extern$);
}

pub fn c_take_callback(callback: fn(String) -> usize) {
    extern "C" {
        #[link_name = "cxxbridge02$c_take_callback"]
        fn __c_take_callback(callback: ::cxx::private::FatFunction);
    }
    let callback = ::cxx::private::FatFunction {
        trampoline: {
            extern "C" {
                #[link_name = "cxxbridge02$c_take_callback$callback$0"]
                fn trampoline();
            }
            #[export_name = "cxxbridge02$c_take_callback$callback$1"]
            unsafe extern "C" fn __(
                _0: *mut ::cxx::private::RustString,
                __extern: fn(String) -> usize,
            ) -> usize {
                let __fn = "cxx_test_suite::ffi::c_take_callback::callback";
                ::cxx::private::catch_unwind(__fn, move || {
                    __extern(::std::mem::take((*_0).as_mut_string()))
                })
            }
            trampoline as usize as *const ()
        },
        ptr: callback as usize as *const (),
    };
    unsafe { __c_take_callback(callback) }
}

---

Part of #52.