p521: initial `ecdsa` support with FIPS 186-4 test vectors #200

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 18s
build (1.65.0, wasm32-unknown-unknown)	3m 29s
build (stable, thumbv7em-none-eabi)	17s
doc	16s
build (stable, wasm32-unknown-unknown)	21s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	4m 17s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 7s
test (x86_64-unknown-linux-gnu, 1.65.0)	4m 2s
test (x86_64-unknown-linux-gnu, stable)	1m 7s
	20m 14s

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