Fix no_std support after updating to latest udigest #349

Job	Run time
check-alloc	6s
check	12s
check-all	15s
build-wasm	3m 53s
build-wasm-nostd	28s
check-doc	17s
check-nostd	6s
check-publish (generic-ec)	15s
check-publish (generic-ec-core)	14s
check-semver (generic-ec, all-features)	2m 41s
check-publish (generic-ec-curves)	9s
check-semver (generic-ec, only-explicit-features)	15s
clippy	13s
check-semver (generic-ec-core, all-features)	13s
check-publish (generic-ec-zkp)	16s
check-semver (generic-ec-core, only-explicit-features)	2m 21s
clippy-nostd	8s
check-semver (generic-ec-curves, all-features)	26s
clippy-tests	16s
check-semver (generic-ec-curves, only-explicit-features)	2m 32s
check-semver (generic-ec-zkp, all-features)	34s
fmt	4s
test	35s
check-semver (generic-ec-zkp, only-explicit-features)	2m 27s
check-changelog	57s
	19m 53s

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