Rework Certificate issuance API, make DER/PEM serialization stable (#… #796

Job	Run time
Validate external types appearing in public API	1m 15s
Format	8s
build-windows	15m 9s
Clippy (--all-features)	4m 41s
Clippy (--no-default-features --features ring)	4m 34s
Check semver compatibility	32s
Clippy (--no-default-features --features aws_lc_rs)	4m 57s
Clippy (--no-default-features --features aws_lc_rs,pem)	4m 52s
Documentation (stable)	34s
build (macOS-latest, stable)	12m 35s
Documentation (beta)	43s
Build rustls-cert-gen as a standalone package	31s
Documentation (nightly)	41s
Measure coverage	5m 39s
Documentation (stable 7 months ago)	50s
build (ubuntu-latest, stable)	5m 41s
build (ubuntu-latest, beta)	5m 47s
build (ubuntu-latest, nightly)	5m 55s
build (ubuntu-latest, stable 7 months ago)	6m 7s
	1h 21m 11s

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