[Relay][Runtime] Add support for virtual machine Objects

[jroesch]

This PR separates the functionality for passing runtime::Objects around from the VM, VM memory manager and VM compiler.

See #2810 and #2889.

[wweic]

lgtm.

[tqchen]

because we do not want the user to touch ptr, consider rename to ptr_,

[tqchen]

Some minor nits. After some thoughts, I personally do not like the fact that Node and Object system are separate. Eventually, we want to merge them into a single node system, but still have a clear separation of compiler and runtime node objects.

For now, we could even reuse the NodeBase class for object(which allows the effective use of make_node). This may need to happen in a few months and I don't want to block the PR, but it might be helpful to keep that in mind when polishing this PR, e.g., the naming(XXXCell-> XXXNode?).

Possible way of unification, for reference:

• NodeBase contains a tag field
• ASTNode subclass NodeBase, and has additional virtual functions for AST level support(attr travseral etc), the tags can dynamically allocated during runtime.
• Object is a subclass of NodeBase, with fixed tags.

[tqchen]

LGTM: unaddress comment: Please rename Object::ptr ->Object::ptr_

[tqchen]

Thanks, @jroesch @wweic @weberlo this is now merged