core: use istr! instead of statically-known ascii_char calls
#27407
| Job | Run time |
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| 5s | |
| 1m 54s | |
| 4m 7s | |
| 5m 26s | |
| 0s | |
| 4m 10s | |
| 0s | |
| 7m 36s | |
| 1m 46s | |
| 3m 1s | |
| 1m 50s | |
| 29m 55s |