Allow pages to be sorted by pinned slot count

By compacting into slots with pinned objects first, we improve the
efficiency of compaction. As it is less likely that there will exist
pages containing only pinned objects after compaction. This will
increase the number of free pages left after compaction and enable us to
free them.

This used to be the default compaction method before it was removed
(inadvertently?) during the introduction of auto_compaction.

This commit will sort the pages by the pinned slot count at the start of
a major GC that has been triggered by explicitly calling GC.compact (and
thus setting objspace->flags.during_compaction).

It works using the same method by which we sort the heap by empty slot
count during GC.verify_compaction_references.

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