Feature: Do not move root objects in Immix

[qinsoon]

This feature would be helpful if a binding allows moving non-root objects, but not root objects. E.g. they are doing conservative stack scanning. The idea is simple: for root edges, we need to tell Immix to mark the objects but not copy them.

[k-sareen]

This could just be implemented by object pinning, right? If you are using conservative roots, then we can just pin the roots.

[qinsoon]

This could just be implemented by object pinning, right? If you are using conservative roots, then we can just pin the roots.

Right. But pinning requires using object metadata for a pinning bit. This does not need extra object metadata.

[k-sareen]

Closed with #897