Add append, add 'equal' to repl

src/builtins.c

@@ -238,3 +238,8 @@ LISP_FUNC_2(ucl_builtin_xor, scope, arg0, arg1) {
 LISP_FUNC_1(ucl_builtin_not, scope, arg0) {
     return ucl_predicate(!ucl_truthy_bool(arg0));
 }
+
+LISP_FUNC_2(ucl_builtin_append, scope, list, elem) {
+    ucl_list_append(list, elem);
+    return list;
+}