Improve performance for unset values and add performance checker example #306

All 190 tests pass in 18s

10 files + 10 10 suites +10 18s ⏱️ +18s
190 tests + 190 190 ✅ + 190 0 💤 ±0 0 ❌ ±0
1 850 runs +1 850 1 850 ✅ +1 850 0 💤 ±0 0 ❌ ±0

Results for commit 6b8cadc. ± Comparison against earlier commit c04a7c6.