Improve GC performance when loading a snapshot

[lemmih]

Issue summary

The semi-space GC loads snapshots into the "new" space rather than the "old" space. This immediately triggers a garbage collection run which isn't necessary and doesn't actually copy anything (yet still wastes a lot of resources). We should load snapshots into the "old" space.

Other information and links

[lemmih]

Depends on #2638

[hanabi1224]

We could maybe add fn is_empty() to Store trait. During GC, when old.is_empty() == true, delete it instead of doing walk_snapshot

[lemmih]

I don't think that is sustainable. Conceptually, we really do want the snapshot data to be placed in the "old" space.

[LesnyRumcajs]

The new, shiny GC (#3504) is on its way, which makes this issue no longer relevant.