sm2

Add `EcdsaCurve` trait impls #338

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	26s
doc	58s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 3s
build (1.73.0, wasm32-unknown-unknown)	30s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 6s
build (stable, thumbv7em-none-eabi)	19s
test (x86_64-unknown-linux-gnu, 1.73.0)	47s
build (stable, wasm32-unknown-unknown)	20s
test (x86_64-unknown-linux-gnu, stable)	38s
	6m 7s

Provide feedback