py/objstringio: Add readinto() method.

Also, drop deprecated (as for MicroPython) readall() method.

py/objstringio.c

@@ -177,7 +177,7 @@ STATIC mp_obj_t stringio_make_new(const mp_obj_type_t *type_in, size_t n_args, s
 
 STATIC const mp_rom_map_elem_t stringio_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_read), MP_ROM_PTR(&mp_stream_read_obj) },
-    { MP_ROM_QSTR(MP_QSTR_readall), MP_ROM_PTR(&mp_stream_readall_obj) },
+    { MP_ROM_QSTR(MP_QSTR_readinto), MP_ROM_PTR(&mp_stream_readinto_obj) },
     { MP_ROM_QSTR(MP_QSTR_readline), MP_ROM_PTR(&mp_stream_unbuffered_readline_obj) },
     { MP_ROM_QSTR(MP_QSTR_write), MP_ROM_PTR(&mp_stream_write_obj) },
     { MP_ROM_QSTR(MP_QSTR_seek), MP_ROM_PTR(&mp_stream_seek_obj) },