p521: implement hash2curve (#964) #123

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 33s
doc	17s
build (1.65.0, wasm32-unknown-unknown)	3m 30s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	4m 43s
build (stable, thumbv7em-none-eabi)	20s
cross (armv7-unknown-linux-gnueabihf, stable)	2m 36s
build (stable, wasm32-unknown-unknown)	17s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	8m 34s
cross (aarch64-unknown-linux-gnu, 1.65.0)	4m 35s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 41s
cross (aarch64-unknown-linux-gnu, stable)	1m 12s
test (x86_64-unknown-linux-gnu, 1.65.0)	4m 16s
cross (powerpc-unknown-linux-gnu, 1.65.0)	6m 55s
test (x86_64-unknown-linux-gnu, stable)	1m 34s
cross (powerpc-unknown-linux-gnu, stable)	1m 12s
	46m 15s

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