Enhance formula processing in `ard_survival_survfit()` #369

All 142 tests pass in 1m 4s

1 files ±0 143 suites ±0 1m 4s ⏱️ -1s
142 tests ±0 142 ✅ ±0 0 💤 ±0 0 ❌ ±0
628 runs ±0 628 ✅ ±0 0 💤 ±0 0 ❌ ±0

Results for commit df57767. ± Comparison against earlier commit f5d4f35.