Replace unsound RefCell usage with UnsafeCell for error buffer
#554
| Job | Run time |
|---|---|
| 20s | |
| 2m 11s | |
| 2m 47s | |
| 2m 48s | |
| 5m 59s | |
| 2m 19s | |
| 2m 26s | |
| 3m 2s | |
| 2m 41s | |
| 2m 37s | |
| 2m 1s | |
| 6m 11s | |
| 5m 7s | |
| 2m 17s | |
| 42m 46s |