p521: get rid of private `MODULUS_HEX` constant (#970) #127

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 32s
build (1.65.0, wasm32-unknown-unknown)	3m 34s
build (stable, thumbv7em-none-eabi)	17s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	8m 48s
build (stable, wasm32-unknown-unknown)	31s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 9s
cross (aarch64-unknown-linux-gnu, 1.65.0)	4m 39s
doc	23s
cross (aarch64-unknown-linux-gnu, stable)	1m 10s
cross (powerpc-unknown-linux-gnu, 1.65.0)	4m 40s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	6m 32s
cross (powerpc-unknown-linux-gnu, stable)	2m 20s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 53s
test (x86_64-unknown-linux-gnu, 1.65.0)	4m 23s
test (x86_64-unknown-linux-gnu, stable)	1m 22s
	46m 13s

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