async-signature: forward `std` feature #2092

Job	Run time
test (1.75.0)	19s
no_std (1.75.0, thumbv7em-none-eabi)	13s
test (stable)	22s
no_std (1.75.0, wasm32-unknown-unknown)	13s
no_std (stable, thumbv7em-none-eabi)	9s
minimal-versions	0s
no_std (stable, wasm32-unknown-unknown)	8s
	1m 24s

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