p521: fiat-constify update #199

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 14s
build (1.65.0, wasm32-unknown-unknown)	1m 10s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	2m 35s
build (stable, thumbv7em-none-eabi)	15s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 33s
build (stable, wasm32-unknown-unknown)	29s
cross (aarch64-unknown-linux-gnu, 1.65.0)	2m 18s
cross (aarch64-unknown-linux-gnu, stable)	1m 20s
doc	21s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	2m 20s
cross (powerpc-unknown-linux-gnu, 1.65.0)	2m 25s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 17s
test (x86_64-unknown-linux-gnu, 1.65.0)	1m 58s
cross (powerpc-unknown-linux-gnu, stable)	1m 22s
test (x86_64-unknown-linux-gnu, stable)	55s
	21m 32s

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