async-signature: `no_std` support (#1544) #1879

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	1m 3s
build (1.60.0, wasm32-unknown-unknown)	1m 4s
build (stable, thumbv7em-none-eabi)	28s
build (stable, wasm32-unknown-unknown)	17s
minimal-versions	35s
test (1.60.0)	1m 12s
test (stable)	19s
	4m 58s

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