p521

p521: initial `ecdsa` support with FIPS 186-4 test vectors #761

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 12s
doc	35s
build (1.65.0, wasm32-unknown-unknown)	50s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	1m 11s
build (stable, thumbv7em-none-eabi)	23s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 5s
build (stable, wasm32-unknown-unknown)	20s
test (x86_64-unknown-linux-gnu, 1.65.0)	1m 2s
test (x86_64-unknown-linux-gnu, stable)	47s
	7m 25s

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