* [RUNTIME] Introduce new object protocol.
This PR introduces a new object protocol to unify the node and object.
We also updated the existing runtime::vm code to make use of the new system.
Update to the node will be done in a follow up PR.
Other changes:
- Remove object related code in json serializer as that code logic was not complete
and we have a separate serializer for VM, can revisit later.
* address review comment
* Fix the child slot logic
Tianqi Chen
a0bd3786ed