`_mlc_address` is too vague name as a unique identifier

[junrushao-openai]

It's known convention in MLC that _mlc_address is used as a unique identifier as oppose to Python's builtin id() for good reasons. However, _mlc_address with an underscore seems less semantically meaningful and less discoverable. We may want to expose two interfaces:

• mlc.Object.id_() which is consistent with Python's id()
• mlc.Object.is_() which is consistent with Python's is

[potatomashed]

Fixed by #57