* [RUNTIME] Refactor object python FFI to new protocol.

This is a pre-req to bring the Node system under object protocol.
Most of the code reflects the current code in the Node system.

- Use new instead of init so subclass can define their own constructors
- Allow register via name, besides type idnex
- Introduce necessary runtime C API functions
- Refactored Tensor and Datatype to directly use constructor.

* address review comments