p521

p521: Update `fiat-crypto` + `fiat-constify` #727

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 44s
doc	28s
build (1.65.0, wasm32-unknown-unknown)	4m 33s
build (stable, thumbv7em-none-eabi)	26s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	52s
build (stable, wasm32-unknown-unknown)	19s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	29s
test (x86_64-unknown-linux-gnu, 1.65.0)	1s
test (x86_64-unknown-linux-gnu, stable)	13s
	12m 5s

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