[bign256] Digital signature #1961

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 35s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	2m 29s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	2m 52s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 35s
doc	28s
build (1.65.0, wasm32-unknown-unknown)	1m 36s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 40s
build (stable, thumbv7em-none-eabi)	35s
test (x86_64-unknown-linux-gnu, 1.65.0)	2m 33s
cross (aarch64-unknown-linux-gnu, 1.65.0)	2m 13s
test (x86_64-unknown-linux-gnu, stable)	1m 22s
build (stable, wasm32-unknown-unknown)	36s
cross (aarch64-unknown-linux-gnu, stable)	1m 15s
cross (powerpc-unknown-linux-gnu, 1.65.0)	2m 17s
cross (powerpc-unknown-linux-gnu, stable)	1m 19s
	24m 25s

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