ICE using parenthetical trait notation in impls 

The following test will ICE:

```
#![feature(unboxed_closures)]
struct Foo;
impl Fn(&int) for Foo { }
```

The workaround for now is to write:

```
impl<'a> Fn(&'a int) for Foo { }
```

It would be better if this were implicit. This seems related to lifetime elision, which also is supposed to permit elided parameters in impls.
