Function pointers should implement the various Fn* types

The following should work:

``` rust
#![feature(unboxed_closures)]
#![feature(overloaded_calls)]

fn action<F: FnOnce() -> ()>(cb: F) {
    cb();
}

fn foo() {
    println!("In foo");
}

pub fn main() {
    action(foo);
}
```
