p521: simplify `FieldElement::from_uint_unchecked` #900

Job	Run time
benches (stable)	1m 9s
build (1.73.0, thumbv7em-none-eabi)	21s
doc	1m 2s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 47s
build (1.73.0, wasm32-unknown-unknown)	22s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 39s
build (stable, thumbv7em-none-eabi)	16s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 26s
test (x86_64-unknown-linux-gnu, stable)	1m 20s
build (stable, wasm32-unknown-unknown)	18s
	9m 40s

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