sm2: add SM2PKE support #315

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	21s
build (1.73.0, wasm32-unknown-unknown)	19s
build (stable, thumbv7em-none-eabi)	29s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 42s
build (stable, wasm32-unknown-unknown)	22s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 37s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 15s
cross (aarch64-unknown-linux-gnu, stable)	1m 17s
doc	1m 2s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 31s
cross (powerpc-unknown-linux-gnu, stable)	1m 24s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 33s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 28s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 9s
test (x86_64-unknown-linux-gnu, stable)	1m 5s
	16m 34s

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