add Object#__tag__

use to detect type of data in internal

```
  TAG_NOT_INITIALIZED: 0x00,
  TAG_OBJ: 0x01,
  TAG_INT: 0x02,
  TAG_SYM: 0x03,
  TAG_CHAR: 0x04,
  TAG_NIL: 0x05,
  TAG_FALSE: 0x06,
  TAG_TRUE: 0x07,
  TAG_PTR: 0x08,
  TAG_FLOAT: 0x09,
  TAG_STR: 0x0a,
  TAG_UNUSED: 0xff,
```