Object Pinning

[qinsoon]

We should add an API for pinning objects, such as fn pin_object(obj: ObjectReference) -> bool, which returns true if the object is pinned, or false if it cannot be pinned.

Whether an object can be pinned depends on its space:

• for spaces such as semispace, objects cannot be pinned (unless we do some heavyweight operations to move the object to a pinnable space).
• for spaces such as marksweep and immix, objects can be pinned if required.
• for spaces such as immortal, objects are naturally pinned, and there is no further action needed to pin the objects.

[qinsoon]

Done in #703. If there is any other issue with object pinning, consider creating a new issue which is more specific to the issue.