Optimize Table::metatable

Author: khvzak

src/table.rs

@@ -487,16 +487,12 @@ impl Table {
     /// [`getmetatable`]: https://www.lua.org/manual/5.4/manual.html#pdf-getmetatable
     pub fn metatable(&self) -> Option<Table> {
         let lua = self.0.lua.lock();
-        let state = lua.state();
+        let ref_thread = lua.ref_thread();
         unsafe {
-            let _sg = StackGuard::new(state);
-            assert_stack(state, 2);
-
-            lua.push_ref(&self.0);
-            if ffi::lua_getmetatable(state, -1) == 0 {
+            if ffi::lua_getmetatable(ref_thread, self.0.index) == 0 {
                 None
             } else {
-                Some(Table(lua.pop_ref()))
+                Some(Table(lua.pop_ref_thread()))
             }
         }
     }