signature: remove `std` feature #2490

Job	Run time
test (1.85.0)	17s
no_std (stable, thumbv7em-none-eabi)	20s
no_std (1.85.0, wasm32-unknown-unknown)	16s
no_std (stable, wasm32-unknown-unknown)	22s
test (stable)	11s
no_std (1.85.0, thumbv7em-none-eabi)	15s
minimal-versions	0s
	1m 41s

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