Implement root_cause using Iterator::last #516

Job	Run time
Test Suite	2m 4s
Test Suite (--no-default-features)	2m 1s
Check (stable)	21s
Test Suite (ubuntu-latest)	24s
Test Suite (--no-default-features --features track-caller)	2m 1s
Test Suite (stable)	1m 6s
Test Suite (windows-latest)	21s
Test Suite (--no-default-features --features auto-install)	2m 5s
Test Suite (stable, --no-default-features)	55s
Test Suite (macOS-latest)	22s
Test Suite (stable, --no-default-features --features track-caller)	50s
Test Suite (stable, --no-default-features --features auto-install)	49s
Clippy (stable)	18s
Test Suite (stable, --features pyo3)	55s
Test Suite (stable, --all-features)	1m 0s
Rustfmt (stable)	5s
Miri	1m 26s
Test Suite (beta)	1m 5s
Test Suite (beta, --no-default-features)	57s
Test Suite (beta, --no-default-features --features track-caller)	58s
Test Suite (beta, --no-default-features --features auto-install)	1m 0s
Test Suite (beta, --features pyo3)	1m 4s
Test Suite (beta, --all-features)	1m 14s
Test Suite (nightly)	1m 5s
Test Suite (nightly, --no-default-features)	1m 0s
Test Suite (nightly, --no-default-features --features track-caller)	1m 2s
Test Suite (nightly, --no-default-features --features auto-install)	1m 0s
Test Suite (nightly, --features pyo3)	1m 3s
Test Suite (nightly, --all-features)	1m 5s
	29m 36s

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