`extern "C"` functions don't implement `Clone`

[bfops]

This should compile. It used to compile without deriving Clone, and it currently compiles if extern "C" is removed.

#[derive(Copy, Clone)]
struct Functions {
  pub foo: extern "C" fn(u32) -> u32,
}

fn main() {
}

[lilyball]

Clone also needs to be implemented for unsafe fn. Right now it's only implemented for fns of 0–8 arguments.

[lilyball]

Looking at the source it's actually explicitly implemented on extern "Rust" fn.

[bfops]

That was quick, thanks!