Allow configuring each Immix space to be non moving

[qinsoon]

We plan to use non-moving Immix as a non-moving space. In this case, we will have two Immix spaces, one allows moving, and the other does not. We currently use global constants to control whether Immix moves objects or not, and we cannot have two Immix spaces that are configured differently.

This PR removes those constants, and adds a flag never_move_objects in ImmixSpaceArgs to indicate whether this Immix space disallows moving.

[qinsoon]

The following is the performance evaluation of this PR. The PR has no measurable impact on performance.

Immix

StickyImmix

GenImmix

[qinsoon]

@wks I have addressed your comments, and posted performance results for the PR. Can you review it again?

[wks]

LGTM

Sorry. There are some issues with PlanConstraints::moves_objects. I'll comment later.

[wks]

One minor issue is that PlanConstraints::moves_objects may conditionally be true if some Cargo features are enabled.

From one hand, we may leave it as true so that the VM will conservatively consider Immix and StickyImmix are moving plans. This is OK for testing object copying on VMs that plans to supported copying GC.

But I think there may be some VMs that intentionally only supports non-copying GCs. They can reject any plans labelled with PlanConstrant::moves_objects == false. In this case, those VM bindings will always enable immix_non_moving and sticky_immix_non_moving_nursery and expect Immix and StickyImmix to have PlanConstrant::moves_objects == true. I am not sure how the performance may be (because of fragmentation), but I think setting PlanConstrant::moves_objects accurately could be helpful.

[wks]

LGTM