Reduce number of calls to Tree.meta in PropagatePositions

Since Tree.meta is property with additional presence check it
introduces unnecessary overhead in accessing underlying Meta
object once it's already initialized. Having local reference to actual
Meta object allows to reduce these presence checks to bare minimum

lark/parse_tree_builder.py

@@ -27,33 +27,39 @@ class PropagatePositions:
     def __call__(self, children):
         res = self.node_builder(children)
 
+        # local reference to Tree.meta reduces number of presence checks
         if isinstance(res, Tree):
+            res_meta = res.meta
             for c in children:
-                if isinstance(c, Tree) and not c.meta.empty:
-                    res.meta.line = c.meta.line
-                    res.meta.column = c.meta.column
-                    res.meta.start_pos = c.meta.start_pos
-                    res.meta.empty = False
-                    break
+                if isinstance(c, Tree):
+                    child_meta = c.meta
+                    if not child_meta.empty:
+                        res_meta.line = child_meta.line
+                        res_meta.column = child_meta.column
+                        res_meta.start_pos = child_meta.start_pos
+                        res_meta.empty = False
+                        break
                 elif isinstance(c, Token):
-                    res.meta.line = c.line
-                    res.meta.column = c.column
-                    res.meta.start_pos = c.pos_in_stream
-                    res.meta.empty = False
+                    res_meta.line = c.line
+                    res_meta.column = c.column
+                    res_meta.start_pos = c.pos_in_stream
+                    res_meta.empty = False
                     break
 
             for c in reversed(children):
-                if isinstance(c, Tree) and not c.meta.empty:
-                    res.meta.end_line = c.meta.end_line
-                    res.meta.end_column = c.meta.end_column
-                    res.meta.end_pos = c.meta.end_pos
-                    res.meta.empty = False
-                    break
+                if isinstance(c, Tree):
+                    child_meta = c.meta
+                    if not child_meta.empty:
+                        res_meta.end_line = child_meta.end_line
+                        res_meta.end_column = child_meta.end_column
+                        res_meta.end_pos = child_meta.end_pos
+                        res_meta.empty = False
+                        break
                 elif isinstance(c, Token):
-                    res.meta.end_line = c.end_line
-                    res.meta.end_column = c.end_column
-                    res.meta.end_pos = c.end_pos
-                    res.meta.empty = False
+                    res_meta.end_line = c.end_line
+                    res_meta.end_column = c.end_column
+                    res_meta.end_pos = c.end_pos
+                    res_meta.empty = False
                     break
 
         return res