`bmain.relations` is used to store temp data of relations between IDs,
to speed-up some complex processes heavily relying on such information.
Previous implementation was failry unclear/confusing, and required a
not-so-nice hack to 'tag' some ID as processed.
New code changes as such:
* Using `from`/`to` naming (instead of `user`/`used`).
* More clear separation between `to` `id_pointer` and `from` one,
using an union instead of hacking around difference between `ID *` and
`ID **` pointers.
* Adds storage of `session_uuid` informations (mainly useful as
debug/ensuring proper consistency of data currently).
* Adds a structure per ID in the mapping. This enables possibility of
storing tags (and potentially more data in the future) per-ID,
without polluting the IDs themselves with very short-life info.
Differential Revision: https://developer.blender.org/D10164
131a758b6f