p521: simplify `FieldElement::from_uint_unchecked` #6

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	16s
build (1.73.0, wasm32-unknown-unknown)	13s
doc	7s
build (stable, thumbv7em-none-eabi)	12s
build (stable, wasm32-unknown-unknown)	9s
test (1.73.0)	40s
test (stable)	10s
	1m 47s

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