p521: simplify `FieldElement::from_uint_unchecked` #1996

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	41s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 43s
doc	56s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 27s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 56s
build (1.73.0, wasm32-unknown-unknown)	41s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 51s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 24s
build (stable, thumbv7em-none-eabi)	45s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 29s
build (stable, wasm32-unknown-unknown)	36s
test (x86_64-unknown-linux-gnu, stable)	1m 22s
cross (aarch64-unknown-linux-gnu, stable)	1m 21s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 41s
cross (powerpc-unknown-linux-gnu, stable)	1m 24s
	19m 17s

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