Implement and add tests for the id() builtin function.
diff --git a/py/builtin.c b/py/builtin.c
index 56cb49d..333119b 100644
--- a/py/builtin.c
+++ b/py/builtin.c
@@ -364,3 +364,9 @@
 }
 
 MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mp_builtin_bytes_obj, 1, 3, mp_builtin_bytes);
+
+static mp_obj_t mp_builtin_id(mp_obj_t o_in) {
+    return mp_obj_new_int_from_uint((machine_uint_t)o_in);
+}
+
+MP_DEFINE_CONST_FUN_OBJ_1(mp_builtin_id_obj, mp_builtin_id);