Merge pull request #28 from ipostelnik/reduce-chrono-features #110

Job	Run time
Docs	2m 38s
Build (stable)	2m 54s
Lints	2m 41s
Build (beta)	2m 54s
Tests (stable)	45s
Build (nightly)	2m 54s
Tests (--no-default-features --features rsa,ecdsa,p256,p384,p521, stable)	52s
Tests (--no-default-features --features fmt, stable)	2m 34s
Tests (--no-default-features --features fmt,ecdsa,p256,p384,p521, stable)	32s
Tests (--no-default-features --features fmt,rsa, stable)	41s
	19m 25s

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