primefield: generic `MontyFieldElement` type #92

Job	Run time
test (stable)	27s
doc	37s
build (stable, thumbv7em-none-eabi)	24s
build (1.85.0, thumbv7em-none-eabi)	28s
build (1.85.0, wasm32-unknown-unknown)	27s
build (stable, wasm32-unknown-unknown)	24s
test (1.85.0)	33s
	3m 20s

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