Allow interpolated paths after `use` during macro expansion.

``` rust
#![feature(macro_rules)]

macro_rules! use_path(($p:path) => (use $p;))

use_path!(std::uint)

fn main() {
    let _ = uint::max_value;
}
```

```
<anon>:3:42: 3:43 error: expected ident, found `std::uint`
<anon>:3 macro_rules! use_path(($p:path) => (use $p;))
                                                  ^
```
