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`.
4c51a66d67