primeorder: use `PrimeField::NUM_BITS` to bound scalar size when mult… #366

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	21s
cross (armv7-unknown-linux-gnueabihf, 1.81.0)	1m 46s
build (1.81.0, wasm32-unknown-unknown)	24s
build (stable, thumbv7em-none-eabi)	13s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 30s
doc	46s
build (stable, wasm32-unknown-unknown)	14s
cross (aarch64-unknown-linux-gnu, 1.81.0)	1m 22s
cross (aarch64-unknown-linux-gnu, stable)	1m 9s
cross (powerpc-unknown-linux-gnu, 1.81.0)	1m 22s
cross (powerpc-unknown-linux-gnu, stable)	1m 20s
test (i686-unknown-linux-gnu, 1.81.0, sudo apt update && sudo apt install gcc-multilib)	1m 40s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 38s
test (x86_64-unknown-linux-gnu, 1.81.0)	1m 8s
test (x86_64-unknown-linux-gnu, stable)	1m 1s
	15m 54s

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