p521: simplify `FieldElement::from_uint_unchecked` #1914

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	39s
build (1.73.0, wasm32-unknown-unknown)	41s
build (stable, thumbv7em-none-eabi)	35s
build (stable, wasm32-unknown-unknown)	36s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 39s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 31s
doc	58s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 34s
cross (aarch64-unknown-linux-gnu, stable)	1m 21s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 49s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 37s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 42s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 24s
cross (powerpc-unknown-linux-gnu, stable)	1m 24s
test (x86_64-unknown-linux-gnu, stable)	1m 17s
	18m 47s

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