p521: switch to upstream RFC6979-based ECDSA #230

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	20s
build (1.73.0, wasm32-unknown-unknown)	23s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 48s
build (stable, thumbv7em-none-eabi)	14s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 34s
build (stable, wasm32-unknown-unknown)	13s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 20s
doc	52s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 24s
cross (aarch64-unknown-linux-gnu, stable)	1m 17s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 20s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 34s
cross (powerpc-unknown-linux-gnu, stable)	1m 19s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 6s
test (x86_64-unknown-linux-gnu, stable)	1m 11s
	15m 55s

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