Blender Git Loki

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

This commit affects `check_for_dupid()` helper:
* Add a special, quicker code path dedicated to sequential addition of a
large number of IDs using the same base name.

This gives a significant speedup to adding 'randomly'-named IDs:

| Number and type of names of IDs | old code | new code | speed improvement |
| -------------------------------- | -------- | -------- | ----------------- |
| 40K, mixed (14k rand, 26k const) | 49s | 39s | 26% |
| 40K, fully random | 51s | 51s | 0% |
| 40K, fully constant | 71s | 40s | 78% |

Note that 'random' names give no improvement as expected, since this new code
path will never be used in such cases.