style: Avoid some allocations in selector serialization.

The allocations in display_to_css_identifier show up in the profiles of
bug 1675628.

Differential Revision: https://phabricator.services.mozilla.com/D97856

components/style/macros.rs

@@ -133,3 +133,10 @@ macro_rules! profiler_label {
 macro_rules! profiler_label {
     ($label_type:ident) => {};
 }
+
+#[cfg(feature = "gecko")]
+macro_rules! local_name {
+    ($s:tt) => {
+        $crate::values::AtomIdent(atom!($s))
+    };
+}