p521

Add `EcdsaCurve` trait impls #910

Job	Run time
benches (stable)	1m 9s
build (1.73.0, thumbv7em-none-eabi)	21s
build (1.73.0, wasm32-unknown-unknown)	21s
doc	1m 7s
build (stable, thumbv7em-none-eabi)	17s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 59s
build (stable, wasm32-unknown-unknown)	28s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 45s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 28s
test (x86_64-unknown-linux-gnu, stable)	1m 26s
	10m 21s

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