k256: use generic signing implementation from `ecdsa` #48

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 17s
build (1.65.0, wasm32-unknown-unknown)	1m 20s
build (stable, thumbv7em-none-eabi)	22s
build (stable, wasm32-unknown-unknown)	14s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	2m 44s
cross (armv7-unknown-linux-gnueabihf, stable)	2m 2s
cross (aarch64-unknown-linux-gnu, 1.65.0)	2m 40s
cross (aarch64-unknown-linux-gnu, stable)	2m 14s
cross (powerpc-unknown-linux-gnu, 1.65.0)	3m 20s
cross (powerpc-unknown-linux-gnu, stable)	1m 52s
doc	23s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	2m 59s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 59s
test (x86_64-unknown-linux-gnu, 1.65.0)	2m 24s
test (x86_64-unknown-linux-gnu, stable)	1m 58s
	27m 48s

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