k256: reject signatures which aren't low-S normalized #1926

Job	Run time
benches (1.65.0)	3m 9s
benches (stable)	2m 37s
build (1.65.0, thumbv7em-none-eabi)	2m 20s
build (1.65.0, wasm32-unknown-unknown)	1m 57s
build (stable, thumbv7em-none-eabi)	1m 4s
build (stable, wasm32-unknown-unknown)	59s
doc	24s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	3m 2s
cross (armv7-unknown-linux-gnueabihf, stable)	2m 34s
cross (aarch64-unknown-linux-gnu, 1.65.0)	3m 14s
cross (aarch64-unknown-linux-gnu, stable)	1m 59s
cross (powerpc-unknown-linux-gnu, 1.65.0)	3m 13s
cross (powerpc-unknown-linux-gnu, stable)	2m 39s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	3m 43s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 38s
test (x86_64-unknown-linux-gnu, 1.65.0)	3m 8s
test (x86_64-unknown-linux-gnu, stable)	2m 4s
	40m 44s

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