[bign256] Digital signature #1955

Job	Run time
benches (1.65.0)	5m 48s
benches (stable)	2m 10s
build (1.65.0, thumbv7em-none-eabi)	4m 33s
doc	30s
build (1.65.0, wasm32-unknown-unknown)	4m 21s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	5m 27s
cross (armv7-unknown-linux-gnueabihf, stable)	2m 2s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	7m 36s
build (stable, thumbv7em-none-eabi)	1m 4s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 47s
cross (aarch64-unknown-linux-gnu, 1.65.0)	6m 41s
build (stable, wasm32-unknown-unknown)	58s
test (x86_64-unknown-linux-gnu, 1.65.0)	6m 42s
cross (aarch64-unknown-linux-gnu, stable)	2m 10s
test (x86_64-unknown-linux-gnu, stable)	2m 26s
cross (powerpc-unknown-linux-gnu, 1.65.0)	6m 14s
cross (powerpc-unknown-linux-gnu, stable)	2m 53s
	1h 4m 22s

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