Support compiling without cryptography primitives #773

Job	Run time
Format	4s
Clippy (--all-features)	34s
Clippy (--no-default-features --features ring)	18s
Clippy (--no-default-features --features aws_lc_rs)	34s
build-windows	8m 10s
Clippy (--no-default-features --features aws_lc_rs,pem)	36s
build (macOS-latest, stable)	5m 30s
Validate external types appearing in public API	1m 21s
Measure coverage	5m 14s
build (ubuntu-latest, stable)	5m 1s
Documentation (stable)	35s
build (ubuntu-latest, beta)	4m 59s
Documentation (beta)	49s
Check semver compatibility	31s
build (ubuntu-latest, nightly)	4m 54s
Documentation (nightly)	44s
build (ubuntu-latest, stable 7 months ago)	4m 54s
Documentation (stable 7 months ago)	1m 0s
	45m 48s

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