sm2: add SM2PKE support #311

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	20s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 48s
build (1.73.0, wasm32-unknown-unknown)	20s
doc	59s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 43s
build (stable, thumbv7em-none-eabi)	23s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 31s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 24s
build (stable, wasm32-unknown-unknown)	20s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 30s
cross (aarch64-unknown-linux-gnu, stable)	1m 22s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 15s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 29s
test (x86_64-unknown-linux-gnu, stable)	1m 10s
cross (powerpc-unknown-linux-gnu, stable)	1m 24s
	16m 58s

Provide feedback