sm2: add SM2PKE support #319

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	23s
build (1.73.0, wasm32-unknown-unknown)	21s
build (stable, thumbv7em-none-eabi)	15s
build (stable, wasm32-unknown-unknown)	17s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 31s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 19s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 12s
test (x86_64-unknown-linux-gnu, stable)	1m 6s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 47s
doc	51s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 37s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 30s
cross (aarch64-unknown-linux-gnu, stable)	1m 18s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 37s
cross (powerpc-unknown-linux-gnu, stable)	1m 22s
	16m 26s

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