k256

Add `EcdsaCurve` trait impls #2130

Job	Run time
benches (1.73.0)	1m 27s
build (1.73.0, thumbv7em-none-eabi)	42s
benches (stable)	1m 25s
build (1.73.0, wasm32-unknown-unknown)	43s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 26s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 32s
build (stable, thumbv7em-none-eabi)	36s
doc	1m 0s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 34s
build (stable, wasm32-unknown-unknown)	44s
cross (aarch64-unknown-linux-gnu, stable)	1m 23s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 37s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 48s
cross (powerpc-unknown-linux-gnu, stable)	1m 22s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 50s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 33s
test (x86_64-unknown-linux-gnu, stable)	1m 31s
	22m 13s

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