primeorder: use `PrimeField::NUM_BITS` to bound scalar size when mult… #145

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	14s
doc	7s
build (1.81.0, wasm32-unknown-unknown)	12s
build (stable, thumbv7em-none-eabi)	9s
test (1.81.0)	10s
build (stable, wasm32-unknown-unknown)	6s
test (stable)	5s
	1m 3s

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