Add more setcoeff! methods and use them to deduplicate PolyRingElem constructors
#3035
| Job | Run time |
|---|---|
| 6m 56s | |
| 1m 59s | |
| 1m 47s | |
| 2m 10s | |
| 2m 23s | |
| 1m 14s | |
| 3m 18s | |
| 19m 47s |