hkdf: automatically switch between `Hmac` and `SimpleHmac` #168

Job	Run time
test (1.72.0)	25s
test (stable)	23s
minimal-versions	0s
build (1.72.0, thumbv7em-none-eabi)	19s
build (1.72.0, wasm32-unknown-unknown)	20s
build (stable, thumbv7em-none-eabi)	25s
build (stable, wasm32-unknown-unknown)	19s
	2m 11s

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