sm2: add SM2PKE support #318

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	20s
doc	57s
build (1.73.0, wasm32-unknown-unknown)	24s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 44s
build (stable, thumbv7em-none-eabi)	14s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 31s
build (stable, wasm32-unknown-unknown)	16s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 37s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 27s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 39s
cross (aarch64-unknown-linux-gnu, stable)	1m 29s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 13s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 36s
test (x86_64-unknown-linux-gnu, stable)	1m 0s
cross (powerpc-unknown-linux-gnu, stable)	1m 19s
	16m 46s

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