p521: Update `fiat-crypto` + `fiat-constify` #1963

Job	Run time
benches (1.65.0)	7m 57s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	8m 38s
build (1.65.0, thumbv7em-none-eabi)	5m 29s
cross (armv7-unknown-linux-gnueabihf, stable)	2m 4s
doc	25s
build (1.65.0, wasm32-unknown-unknown)	5m 16s
cross (aarch64-unknown-linux-gnu, 1.65.0)	6m 26s
benches (stable)	2m 33s
build (stable, thumbv7em-none-eabi)	50s
cross (aarch64-unknown-linux-gnu, stable)	2m 30s
build (stable, wasm32-unknown-unknown)	1m 7s
cross (powerpc-unknown-linux-gnu, 1.65.0)	6m 34s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	7m 34s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 43s
cross (powerpc-unknown-linux-gnu, stable)	2m 12s
test (x86_64-unknown-linux-gnu, 1.65.0)	6m 52s
test (x86_64-unknown-linux-gnu, stable)	2m 33s
	1h 11m 43s

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