Add feature flag for enabling FIPS. #432

Job	Run time
Check for documentation errors	46s
Clippy	49s
Build+test (stable, ubuntu-20.04)	2m 16s
features	1m 39s
Build+test (beta, ubuntu-20.04)	2m 20s
Format	14s
msrv	1m 39s
Build+test (nightly, ubuntu-20.04)	2m 19s
Build+test (macos-latest, stable)	53m 11s
Check semver compatibility	3m 6s
Build+test (windows-latest, stable)	6m 51s
	1h 15m 10s

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