primefield: generic `MontyFieldElement` type #86

Job	Run time
test (stable)	32s
build (stable, thumbv7em-none-eabi)	30s
build (1.85.0, thumbv7em-none-eabi)	28s
build (stable, wasm32-unknown-unknown)	24s
build (1.85.0, wasm32-unknown-unknown)	23s
test (1.85.0)	29s
doc	38s
	3m 24s

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