Add more setcoeff! methods and use them to deduplicate PolyRingElem constructors
#2901
| Job | Run time |
|---|---|
| 16m 51s | |
| 1m 51s | |
| 1m 48s | |
| 1m 57s | |
| 2m 33s | |
| 2m 43s | |
| 1m 17s | |
| 3m 15s | |
| 32m 15s |