p521: Update `fiat-crypto` + `fiat-constify` #1849

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	6m 35s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	8m 9s
build (1.65.0, wasm32-unknown-unknown)	5m 13s
cross (armv7-unknown-linux-gnueabihf, stable)	2m 52s
build (stable, thumbv7em-none-eabi)	45s
cross (aarch64-unknown-linux-gnu, 1.65.0)	7m 9s
build (stable, wasm32-unknown-unknown)	43s
cross (aarch64-unknown-linux-gnu, stable)	2m 23s
cross (powerpc-unknown-linux-gnu, 1.65.0)	6m 37s
doc	33s
cross (powerpc-unknown-linux-gnu, stable)	2m 19s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	8m 53s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 47s
test (x86_64-unknown-linux-gnu, 1.65.0)	8m 30s
test (x86_64-unknown-linux-gnu, stable)	2m 20s
	1h 5m 48s

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