async-signature: `no_std` support (#1544) #2707

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	37s
minimal-versions	0s
test (1.73.0)	30s
build (1.73.0, wasm32-unknown-unknown)	41s
test (stable)	34s
build (stable, thumbv7em-none-eabi)	39s
build (stable, wasm32-unknown-unknown)	40s
test (nightly)	32s
	4m 13s

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