* [REFACTOR][NODE][RUNTIME] Move Node to the new Object protocol.

This PR removes the original node system, and make node as a subclass of Object.
This is a major refactor towards a better unified runtime object system.

List of changes in the refactor:

- We now hide data_ field, use Downcast explicitly to get a sub-class object.
- Removed the node system FFI in python.
- Removed the node C API, instead use PackedFunc for list and get attrs.
- Change relay::Op::set_attr_type_key(attr_key_name) to relay::Op::set_attr_type<AttrType>().
  - This change was necessary because of the new Object registration mechanism.
  - Subsequent changes to the op registrations
  - The change revealed a few previous problems that is now fixed.
- Patched up a few missing node type registration.
  - Now we will raise an error if we register object that is not registered.
- The original node.h and container.h are kept in the same location.
- Calling convention: kObjectHandle now equals the old kNodeHandle, kNodeHandle is removed.
- IRFunctor now dispatches on ObjectRef.
- Update to the new type checking API: is_type, derived_from are replaced by IsInstance.
- Removed .hash member function, instead use C++ convention hasher functors.

* Address review comments

* [HEADER] ASF header dir=include

* [HEADER] ASF Header dir=src

* [HEADER] ASF Header -dir=python

* [HEADER] ASF header dir=topi

* [HEADER] ASF Header dir=nnvm

* [HEADER] ASF Header -dir=tutorials

* [HEADER] ASF Header dir=tests

* [HEADER] ASF Header -dir=docker

* fix whitespace

* [HEADER] ASF Header -dir=jvm

* [HEADER] ASF Header -dir=web

* [HEADER] ASF Header --dir=apps

* [HEADER] ASF Header --dir=vta

* [HEADER] ASF Header -dir=go

* temp

* [HEADER] ASF Header --dir=rust

* [HEADER] Add ASF Header --dir=cmake

* [HEADER] ASF Header --dir=docs

* [HEADER] Header for Jenkinsfile

* [HEADER] ASF Header to toml and md

* [HEADER] ASF Header to gradle

* Finalize rat cleanup

* Fix permission

* Fix java test

* temporary remove nnvm onnx test

* Add same_as to NodeBase

1. Most class inherited from NodeBase(Schedule, Stage, etc) still have
the convenience of using '==' for object identity. And this is the right
behavior for non-Expr classes.
2. subclasses of ExprOp now create EQ expression when '==' is used.

`__nonzero__` and `__bool__` in EQ and NE is a comprise that in some cases
object identity semantics is still useful, like in unit test. For instance:
````
assert a == b
````

"a == b" will create EQ expression, assert then calls `__nonzero__` of the
result expression. `Expr.__nonzero__` throws exception since it prohibits
evaluating IR expression.

More complex case like:
````
assert a in b # b is dict
````

it will call `__eq__` on a and all keys of b, then `__bool__` on the result
expression. This could not easily be done by same_as.

* Retain __hash__ from NodeBase in Python3

* [C API] Make DSL API registerable, add copy from/to raw bytes

* fix cython

* [FIX] Miss kUInt in TypeCode2Str & dir method

* [FIX] Add regression test

* [PYTHON/FFI] Enable Cython FFI

* fix cython

* [DOC] API doc organization.

* remove breathe for now

* [DOC] Initial doc system

* Migrate API

* Update docs