Blender Git Loki

ID Management: Improve speed of code used when creating/renaming and ID.

This commit affects `id_sort_by_name()` and `check_for_dupid()` helper:
* Add a new parameter, `ID *id_sorting_hint`, to `id_sort_by_name()`,
and when non-NULL, check if we can insert `id` immediately before or
after it. This can dramatically reduce time spent in that function.
* Use loop over whole list in `check_for_dupid()` to also define the
likely ID pointer that will be neighbor with our new one.

This gives another decent speedup to all massive addition cases:

| Number and type of names of IDs | old code | new code | speed improvement |
| -------------------------------- | -------- | -------- | ----------------- |
| 40K, mixed (14k rand, 26k const) | 39s | 33s | 18% |
| 40K, fully random | 51s | 42s | 21% |
| 40K, fully constant | 40s | 34s | 18% |

Combined with the previous commits, this makes massive addition of IDs more
than twice as fast as previously.