Improve performance for unset values and add performance checker example #310

All 190 tests pass in 17s

10 files + 10 10 suites +10 17s ⏱️ +17s
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 ae872ac. ± Comparison against earlier commit 8a8d645.