Elide private key in KeyPair Debug impl #1767

Job	Run time
Check semver compatibility	3m 14s
Build rustls-cert-gen as a standalone package	32s
build (macOS-latest, stable)	4m 25s
Validate external types appearing in public API	1m 46s
build (ubuntu-latest, stable)	6m 53s
build-windows	13m 53s
build (ubuntu-latest, beta)	7m 0s
Measure coverage	7m 6s
build (ubuntu-latest, nightly)	6m 44s
Format & clippy	6m 54s
build (ubuntu-latest, stable 7 months ago)	7m 12s
Documentation (stable)	1m 20s
Documentation (nightly)	1m 14s
	1h 8m 13s

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