Tests

Add ToPrettyString; clean up fnvalue tests #1030

Job	Run time
build (macOS-latest, stable)	2m 50s
build (macOS-latest, nightly)	3m 15s
build (macOS-latest, 1.70)	3m 57s
build (ubuntu-latest, stable)	2m 5s
build (ubuntu-latest, nightly)	2m 28s
build (ubuntu-latest, 1.70)	3m 2s
build (windows-latest, stable)	7m 15s
build (windows-latest, nightly)	5m 35s
build (windows-latest, 1.70)	5m 53s
cargo-mutants	2h 25m 20s
	3h 1m 40s

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