Don't share id_map and deref_id_map

[camelid]

All the tests passed, so it doesn't seem they need to be shared.
Plus they should be item/page-specific.

I'm not sure why they were shared before. I think the reason id_map
worked as a shared value before is that it is cleared before rendering
each item (in render_item). And then I'm guessing deref_id_map
worked because it's a hashmap keyed by DefId, so there was no overlap
(though I'm guessing we could have had issues in the future).

Note that id_map currently still has to be cleared because otherwise
child items would inherit the id_map of their parent. I'm hoping to
figure out a way to stop cloning Context, but until then we have to
reset id_map.

Builds on top of #82356 (otherwise we'd have merge conflicts), so this is
blocked on that PR.