Can't find impl for type involving &

```
rusti> trait Tr { fn foo(&self); }
()
rusti>  impl Tr for &int { fn foo(&self) {} }
()
rusti> fn foo<T: Tr>(a: &T) { a.foo(); }
()
rusti> foo(&5);
<anon>:45:0: 45:3 error: failed to find an implementation of trait @main::Tr for <V1>
```

There are similarly confusing errors involving unsafe pointers, but I haven't figured out a good testcase yet.
