Make ring optional #509

Job	Run time
Clippy	36s
Measure coverage	40s
Check cross compilation targets	1m 29s
Cargo Deny	1m 18s
MSRV	1m 15s
Cargo Package	30s
Check for documentation errors (stable)	31s
Check for documentation errors (beta)	29s
Check for documentation errors (nightly)	25s
Format	16s
Check semver compatibility	1m 58s
Build+test (--all-features, stable)	1m 2s
Build+test (stable, ubuntu-20.04)	1m 18s
Build+test (--features=alloc, stable, ubuntu-20.04)	42s
Build+test (--no-default-features, stable, ubuntu-20.04)	35s
Build+test (--all-features, --release, stable, ubuntu-20.04)	2m 5s
Build+test (--all-features, nightly, ubuntu-20.04)	32s
Build+test (--all-features, beta, ubuntu-20.04)	31s
	16m 12s

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