Rationale: When the `revoke` handler is called for a particular
        cspace, cnode, and object, the cptr associated with that cnode
        needs to be free'd so it can be re-used by other objects (given
        there are a relatively small number of cptrs in a cspace).
        We can discover the cptr_cache associated with a cspace by
        looking at the `cspace->owner` field (which we point at our
        cache), but we can't figure out from the cnode alone which
        pointer in that cache points to the given cnode. This
        information is known when the cnode is created, so I added a
        field to the cnode that contains this information.