k256

Add `EcdsaCurve` trait impls #2129

Job	Run time
benches (1.73.0)	1m 22s
build (1.73.0, thumbv7em-none-eabi)	28s
benches (stable)	1m 22s
build (1.73.0, wasm32-unknown-unknown)	29s
build (stable, thumbv7em-none-eabi)	27s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 33s
build (stable, wasm32-unknown-unknown)	29s
doc	1m 1s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 29s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 32s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	2m 1s
cross (aarch64-unknown-linux-gnu, stable)	1m 35s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 33s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 52s
cross (powerpc-unknown-linux-gnu, stable)	1m 18s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 41s
test (x86_64-unknown-linux-gnu, stable)	1m 24s
	21m 36s

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