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

lang/ast/selector.c

@@ -215,7 +215,9 @@ static void print(struct ivy_ast_node *node)
 		fputc('-', stdout);
 		break;
 	default:
-		fputc('?', stdout);
+		/* this will occur if the selector is being used to send a
+		   message at runtime, rather than as part of a message
+		   handler definition. */
 		break;
 	}