p521: initial `ecdsa` support with FIPS 186-4 test vectors #102

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 25s
build (1.65.0, wasm32-unknown-unknown)	3m 37s
doc	27s
build (stable, thumbv7em-none-eabi)	13s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	4m 31s
build (stable, wasm32-unknown-unknown)	27s
cross (armv7-unknown-linux-gnueabihf, stable)	2m 25s
cross (aarch64-unknown-linux-gnu, 1.65.0)	4m 31s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	8m 0s
cross (aarch64-unknown-linux-gnu, stable)	1m 8s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 18s
cross (powerpc-unknown-linux-gnu, 1.65.0)	4m 28s
test (x86_64-unknown-linux-gnu, 1.65.0)	4m 15s
cross (powerpc-unknown-linux-gnu, stable)	1m 17s
test (x86_64-unknown-linux-gnu, stable)	57s
	41m 59s

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