lang: ast: convert RPN expression queue to ast and replace generic EXPR node with it

lang/include/ivy/lang/ast.h

@@ -63,7 +63,7 @@ struct ivy_ast_unit_node {
 
 struct ivy_ast_op_node {
 	struct ivy_ast_node n_base;
-	enum ivy_operator_id n_op;
+	const struct ivy_operator *n_op;
 	struct ivy_ast_node *n_left;   // NULL for unary operators.
 	struct ivy_ast_node *n_right;
 };