Foreign function visible outside of crate.

``` rust
// lib.rs
#![crate_name="foo"]
#![crate_type="lib"]

extern {
    fn bar();
}

pub fn visible() {
}

fn invisible() {
}
```

``` rust
// example.rs
extern crate foo;

fn main() {
    foo::visible();
    foo::bar(); // visible
//  foo::invisible();
}
```

$ rustc --version
rustc 0.12.0-pre-nightly (75396b2a0 2014-08-22 23:55:50 +0000)
