p224

p521: simplify `FieldElement::from_uint_unchecked` #536

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	29s
build (1.73.0, wasm32-unknown-unknown)	30s
build (stable, thumbv7em-none-eabi)	31s
build (stable, wasm32-unknown-unknown)	29s
doc	59s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	55s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	47s
test (x86_64-unknown-linux-gnu, 1.73.0)	38s
test (x86_64-unknown-linux-gnu, stable)	34s
	5m 52s

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