Refactor `KeyPair` constructor #484

Job	Run time
build-windows	11m 9s
Measure coverage	1m 0s
Clippy	25s
Documentation (stable)	39s
Format	8s
Documentation (beta)	40s
Documentation (nightly)	40s
build (macOS-latest, stable)	46s
Documentation (stable 7 months ago)	4m 7s
build (ubuntu-latest, stable)	8m 17s
build (ubuntu-latest, beta)	11m 33s
build (ubuntu-latest, nightly)	12m 8s
build (ubuntu-latest, stable 7 months ago)	1m 11s
	52m 43s

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