p521: initial `ecdsa` support with FIPS 186-4 test vectors #206

Job	Run time
doc	30s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	4m 55s
build (1.65.0, thumbv7em-none-eabi)	6m 34s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	54s
build (1.65.0, wasm32-unknown-unknown)	5m 21s
test (x86_64-unknown-linux-gnu, 1.65.0)	6m 17s
build (stable, thumbv7em-none-eabi)	35s
test (x86_64-unknown-linux-gnu, stable)	24s
build (stable, wasm32-unknown-unknown)	19s
	25m 49s

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