Lifetime of fn() converted to &Fn() isn't long enough

``` rust
fn double(x: i32) -> i32 { x * 2 }
fn get_double_func() -> &'static (Fn(i32) -> i32)
{
    // Works
    const GET_DOUBLE_FUNC: &'static (Fn(i32) -> i32) = &double;
    GET_DOUBLE_FUNC
}
fn get_double_func_2() -> &'static (Fn(i32) -> i32)
{
    // Error message
    &double
}
fn main() {}
```

It would be nice if the lifetime of `&double` were the same in all contexts.
