Add performant implementations of get_id, has_class, and each_class.

ports/geckolib/string_cache/lib.rs

@@ -134,6 +134,10 @@ impl Atom {
         self.0
     }
 
+    pub unsafe fn with<F>(ptr: *mut nsIAtom, callback: &mut F) where F: FnMut(&Atom) {
+        callback(transmute(&ptr))
+    }
+
     // Static atoms have a dummy AddRef/Release, so we don't bother calling
     // AddRef() here. This would cause memory corruption with non-static atoms
     // both because (a) we wouldn't hold the atom alive, and (b) we can't avoid
@@ -226,3 +230,12 @@ impl From<String> for Atom {
         Atom::from(&*string)
     }
 }
+
+impl From<*mut nsIAtom> for Atom {
+    fn from(ptr: *mut nsIAtom) -> Atom {
+        unsafe {
+            Gecko_AddRefAtom(ptr);
+            Atom(ptr)
+        }
+    }
+}