p521

p384: remove default `ecdh` feature (#1112) #1038

Job	Run time
benches (stable)	1m 11s
build (1.81.0, thumbv7em-none-eabi)	19s
build (1.81.0, wasm32-unknown-unknown)	19s
build (stable, thumbv7em-none-eabi)	21s
doc	56s
build (stable, wasm32-unknown-unknown)	23s
test (i686-unknown-linux-gnu, 1.81.0, sudo apt update && sudo apt install gcc-multilib)	1m 42s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 45s
test (x86_64-unknown-linux-gnu, 1.81.0)	1m 17s
test (x86_64-unknown-linux-gnu, stable)	1m 21s
	9m 34s

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