Allow interpolated paths after `use` during macro expansion.

[brendanzab]

#![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;))
                                                  ^

[huonw]

Triage, the error message has improved slightly, but no change (I updated the example code & diagnostics). It's worth noting that import paths are actually different to normal paths, since e.g. foo::<T>::bar::<U> is a valid path, but not a valid import path.

(For completeness, I believe there's no import paths that are invalid normal paths, e.g. super::super::foo() is a valid expression.)

[steveklabnik]

Triage: no change