k256

Add ECDH + PKCS8 to bign256 #2167

Job	Run time
benches (1.73.0)	1m 20s
build (1.73.0, thumbv7em-none-eabi)	31s
benches (stable)	1m 5s
build (1.73.0, wasm32-unknown-unknown)	31s
build (stable, thumbv7em-none-eabi)	21s
build (stable, wasm32-unknown-unknown)	1s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 25s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 25s
doc	1m 1s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 28s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	38s
cross (aarch64-unknown-linux-gnu, stable)	1m 12s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	36s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 23s
test (x86_64-unknown-linux-gnu, 1.73.0)	40s
cross (powerpc-unknown-linux-gnu, stable)	1m 16s
test (x86_64-unknown-linux-gnu, stable)	26s
	15m 19s

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