p521: make `LooseFieldElement` pub (#978) #163

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 43s
build (1.65.0, wasm32-unknown-unknown)	4m 4s
build (stable, thumbv7em-none-eabi)	14s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	4m 42s
build (stable, wasm32-unknown-unknown)	15s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 21s
cross (aarch64-unknown-linux-gnu, 1.65.0)	4m 48s
doc	13s
cross (aarch64-unknown-linux-gnu, stable)	1m 10s
cross (powerpc-unknown-linux-gnu, 1.65.0)	4m 43s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	4m 52s
cross (powerpc-unknown-linux-gnu, stable)	1m 17s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 12s
test (x86_64-unknown-linux-gnu, 1.65.0)	4m 27s
test (x86_64-unknown-linux-gnu, stable)	49s
	37m 50s

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