Unboxed closure: infer `move` when passing an unboxed closure to Fn* + Send

It would be really amazing if in the following snippet, the "move" was inferred by the fact that the closure is passed as a Fn\* + Send. Aka, that move was inferred by the Send trait.

```
fn stuff<F: FnOnce() + Send>(f: F) {
}

pub fn main() {
    stuff(move |:| println!("hello"));
}
```
