p521: add `ecdh` feature (#954) #98

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 17s
build (1.65.0, wasm32-unknown-unknown)	4m 51s
build (stable, thumbv7em-none-eabi)	12s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	6m 25s
build (stable, wasm32-unknown-unknown)	27s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 13s
cross (aarch64-unknown-linux-gnu, 1.65.0)	7m 33s
doc	15s
cross (aarch64-unknown-linux-gnu, stable)	1m 10s
cross (powerpc-unknown-linux-gnu, 1.65.0)	6m 32s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	6m 51s
cross (powerpc-unknown-linux-gnu, stable)	1m 56s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 12s
test (x86_64-unknown-linux-gnu, 1.65.0)	6m 18s
test (x86_64-unknown-linux-gnu, stable)	1m 26s
	50m 38s

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