p521: Update `fiat-crypto` + `fiat-constify` #1771

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 20s
build (1.65.0, wasm32-unknown-unknown)	6m 18s
build (stable, thumbv7em-none-eabi)	1m 3s
build (stable, wasm32-unknown-unknown)	59s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	10m 42s
cross (armv7-unknown-linux-gnueabihf, stable)	2m 11s
doc	38s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	7m 24s
cross (aarch64-unknown-linux-gnu, 1.65.0)	6m 38s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 38s
cross (aarch64-unknown-linux-gnu, stable)	2m 41s
test (x86_64-unknown-linux-gnu, 1.65.0)	6m 45s
cross (powerpc-unknown-linux-gnu, 1.65.0)	7m 58s
test (x86_64-unknown-linux-gnu, stable)	2m 10s
cross (powerpc-unknown-linux-gnu, stable)	2m 4s
	1h 5m 29s

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