Support compiling without cryptography primitives #800

Job	Run time
Validate external types appearing in public API	1m 16s
build-windows	9m 31s
build (macOS-latest, stable)	55s
Build rustls-cert-gen as a standalone package	33s
build (ubuntu-latest, stable)	5m 41s
Clippy (--all-features)	5m 5s
build (ubuntu-latest, beta)	43s
Measure coverage	40s
Clippy (--no-default-features --features ring)	4m 30s
build (ubuntu-latest, nightly)	5m 27s
Clippy (--no-default-features --features aws_lc_rs)	4m 45s
Documentation (stable)	45s
build (ubuntu-latest, stable 7 months ago)	52s
Clippy (--no-default-features --features aws_lc_rs,pem)	4m 37s
Documentation (beta)	43s
Format	6s
Documentation (nightly)	42s
Check semver compatibility	30s
Documentation (stable 7 months ago)	49s
	48m 10s

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