Add framework for the union all logic for labels

I am going to need to write the union all part myself and take out
the inheritance stuff. For both the ltree query part and to be able
to partition labels in the future.

Now I just need to find a way to support Prepared Statements...

Author: JoshInnis

src/backend/nodes/ag_nodes.c

@@ -66,7 +66,8 @@ const char *node_names[] = {
     "cypher_merge_information",
     "cypher_create_graph",
     "cypher_use_graph",
-    "cypher_drop_graph"
+    "cypher_drop_graph",
+    "cypher_label_tree_node"
 };
 
 /*
@@ -135,7 +136,8 @@ const ExtensibleNodeMethods node_methods[] = {
     DEFINE_NODE_METHODS_EXTENDED(cypher_merge_information),
     DEFINE_NODE_METHODS(cypher_create_graph),
     DEFINE_NODE_METHODS(cypher_use_graph),
-    DEFINE_NODE_METHODS(cypher_drop_graph)
+    DEFINE_NODE_METHODS(cypher_drop_graph),
+    DEFINE_NODE_METHODS(cypher_label_tree_node)
 };
 
 static bool equal_ag_node(const ExtensibleNode *a, const ExtensibleNode *b)

src/backend/nodes/cypher_outfuncs.c

@@ -451,6 +451,14 @@ void out_cypher_use_graph(StringInfo str, const ExtensibleNode *node)
     WRITE_STRING_FIELD(graph_name);
 }
 
+void out_cypher_label_tree_node(StringInfo str, const ExtensibleNode *node)
+{
+    DEFINE_AG_NODE(cypher_label_tree_node);
+
+    WRITE_NODE_FIELD(larg);
+    WRITE_NODE_FIELD(rarg);
+    WRITE_STRING_FIELD(label_name);
+}
 
 void out_cypher_drop_graph(StringInfo str, const ExtensibleNode *node)
 {