Manual implementation of a `Fn*` trait.

Erroneous code example:

```compile_fail,E0183
struct MyClosure {
    foo: i32
}

impl FnOnce<()> for MyClosure {  // error
    type Output = ();
    extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
        println!("{}", self.foo);
    }
}
```

Manually implementing `Fn`, `FnMut` or `FnOnce` is unstable
and requires `#![feature(fn_traits, unboxed_closures)]`.

```
#![feature(fn_traits, unboxed_closures)]

struct MyClosure {
    foo: i32
}

impl FnOnce<()> for MyClosure {  // ok!
    type Output = ();
    extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
        println!("{}", self.foo);
    }
}
```

The arguments must be a tuple representing the argument list.
For more info, see the [tracking issue][iss29625]:

[iss29625]: https://github.com/rust-lang/rust/issues/29625