p521: initial `ecdsa` support with FIPS 186-4 test vectors #1789

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 40s
build (1.65.0, wasm32-unknown-unknown)	3m 52s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	10m 9s
build (stable, thumbv7em-none-eabi)	57s
build (stable, wasm32-unknown-unknown)	40s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 35s
doc	23s
cross (aarch64-unknown-linux-gnu, 1.65.0)	9m 3s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	5m 31s
cross (aarch64-unknown-linux-gnu, stable)	1m 22s
cross (powerpc-unknown-linux-gnu, 1.65.0)	4m 41s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	3m 28s
cross (powerpc-unknown-linux-gnu, stable)	1m 25s
test (x86_64-unknown-linux-gnu, 1.65.0)	4m 51s
test (x86_64-unknown-linux-gnu, stable)	1m 20s
	54m 57s

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