Remove old forward declaration.
Improve some id_manager comments.
Clean header id generation.
Clean final id generation.
Extract 'order' from `id_placeholder`.
Extract `id_data` from `id_placeholder`.
Combine 'child' and 'unresolved' id states.
Pull the resolved state from `id_placeholder`.
Stop storing generated ids in `id_placeholder`.

Placeholders are now completely constant.
Improve the handling of file ids.

Instead of messing around with the section's id, leave it alone and
check for a file id whenever looking up the current id.
Mark up const use of `id_state` and `id_placeholder`.
Make entity data const where possible.
Split up the id manager file.