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.

[wks]

This line never_move_objects: cfg!(feature = "immix_non_moving") is repeated three times for Immix, GenImmix and StickyImmix. But the semantics of the Cargo feature "immix_non_moving" is disallowing any ImmixSpace instance to move objects. Repeating cfg!(feature = "immix_non_moving") imposes a danger that in the future, as we add more spaces or plans, we may miss one or two of this, and accidentally assign true to never_move_objects when the "immix_non_moving" feature is enabled.

I suggest we allow plans to set never_move_objects according to the requirements in the "normal" scenario (i.e. the main ImmixSpace is movable, but the dedicated non-moving space is non-movable), and let ImmixSpace::new override its value according to cfg!(feature = "immix_non_moving"). In this way, we guarantee never_move_objects == false if the "immix_non_moving" feature is enabled.

Suggestion:

Suggested change

	never_move_objects: cfg!(feature = "immix_non_moving"),
	never_move_objects: true,

But override it in ImmixSpace::new.

[wks]

We can make space_args mutable

-space_args: ImmixSpaceArgs,
+mut space_args: ImmixSpaceArgs,

and override never_move_objects here if cfg!(feature = "immix_non_moving") is true. We can also print a log here, telling the user that this is overridden.

[wks]

This existing info! log here was intended to warn the user that the user is forcing the ImmxSpace not to move objects, which may hurt the performance. We can still print this log if the user overrides never_move_objects with cfg!(feature = "immix_non_moving"). Maybe raise it to the warn! level.

But if the current ImmixSpace is the dedicated non-moving space, it may not be worth logging this at the info! level because it is a normal thing.