Parse unique box types

Issue #409

src/comp/syntax/print/pprust.rs

@@ -260,6 +260,7 @@ fn print_type(s: ps, ty: @ast::ty) {
       ast::ty_char. { word(s.s, "char"); }
       ast::ty_str. { word(s.s, "str"); }
       ast::ty_box(mt) { word(s.s, "@"); print_mt(s, mt); }
+      ast::ty_uniq(mt) { word(s.s, "~"); print_mt(s, mt); }
       ast::ty_vec(mt) {
         word(s.s, "[");
         alt mt.mut {