Fix no_std support after updating to latest udigest #350

Job	Run time
check-alloc	6s
check-all	15s
check	6s
check-nostd	11s
test	37s
build-wasm-nostd	1m 28s
check-changelog	3s
build-wasm	40s
check-doc	20s
check-publish (generic-ec)	20s
check-semver (generic-ec, all-features)	30s
check-publish (generic-ec-core)	1s
check-semver (generic-ec, only-explicit-features)	37s
clippy	1s
check-semver (generic-ec-core, all-features)	20s
check-semver (generic-ec-core, only-explicit-features)	1s
clippy-nostd	13s
check-publish (generic-ec-curves)	14s
check-semver (generic-ec-curves, all-features)	21s
check-publish (generic-ec-zkp)	12s
clippy-tests	12s
fmt	6s
check-semver (generic-ec-curves, only-explicit-features)	12s
check-semver (generic-ec-zkp, all-features)	27s
check-semver (generic-ec-zkp, only-explicit-features)	16s
	7m 49s

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