p521: simplify `FieldElement::from_uint_unchecked` #331

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	28s
build (1.73.0, wasm32-unknown-unknown)	27s
build (stable, thumbv7em-none-eabi)	19s
build (stable, wasm32-unknown-unknown)	20s
doc	52s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	54s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	49s
test (x86_64-unknown-linux-gnu, 1.73.0)	34s
test (x86_64-unknown-linux-gnu, stable)	33s
	5m 16s

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