Optimize Montgomery multiplication #930

Job	Run time
Clippy	48s
Docs	52s
Cargo deny check	21s
Feature checks	30s
Fmt	12s
test (stable, --no-default-features)	57s
test (stable)	1m 14s
test (stable, --all-features)	2m 11s
test (beta, --no-default-features)	1m 0s
test (beta)	1m 1s
test (beta, --all-features)	2m 20s
test (nightly, --no-default-features)	54s
test (nightly)	58s
test (nightly, --all-features)	2m 5s
test (1.65, --no-default-features)	45s
test (1.65)	46s
Accept	1s
	16m 55s

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