Blender Git Loki

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

This commit affects `check_for_dupid()` helper:
* Further simplify the general logic of the code (we now typically only do
one loop over the list of data-blocks, instead of two).

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) | 62s | 49s | 27% |
| 40K, fully random | 76s | 51s | 49% |
| 40K, fully constant | 77s | 71s | 8% |

Note that 'constant' names give little improvement, as in that case the first
loop over the list of IDs (checking whether base given name was already in use)
was aborting very quickly.