Rework Certificate issuance API, make DER/PEM serialization stable #767

Job	Run time
Validate external types appearing in public API	1m 15s
build-windows	16m 55s
build (macOS-latest, stable)	12m 52s
Clippy (--all-features)	4m 49s
build (ubuntu-latest, stable)	6m 2s
Clippy (--no-default-features --features ring)	4m 40s
Measure coverage	5m 53s
build (ubuntu-latest, beta)	6m 8s
Documentation (stable)	1m 7s
Clippy (--no-default-features --features aws_lc_rs)	4m 43s
build (ubuntu-latest, nightly)	5m 55s
Documentation (beta)	41s
Clippy (--no-default-features --features aws_lc_rs,pem)	4m 46s
Documentation (nightly)	42s
Format	5s
build (ubuntu-latest, stable 7 months ago)	6m 24s
Documentation (stable 7 months ago)	47s
Check semver compatibility	32s
	1h 24m 16s

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