p521: implement hash2curve #117

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 31s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	8m 1s
build (1.65.0, wasm32-unknown-unknown)	6m 26s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 43s
build (stable, thumbv7em-none-eabi)	16s
cross (aarch64-unknown-linux-gnu, 1.65.0)	4m 45s
build (stable, wasm32-unknown-unknown)	17s
cross (aarch64-unknown-linux-gnu, stable)	1m 11s
cross (powerpc-unknown-linux-gnu, 1.65.0)	8m 47s
doc	38s
cross (powerpc-unknown-linux-gnu, stable)	1m 22s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	8m 27s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 15s
test (x86_64-unknown-linux-gnu, 1.65.0)	4m 19s
test (x86_64-unknown-linux-gnu, stable)	55s
	51m 53s

Provide feedback