Optimize Montgomery multiplication #927

Job	Run time
Clippy	49s
Feature checks	17s
Docs	51s
Cargo deny check	19s
Fmt	15s
test (stable, --no-default-features)	1m 37s
test (stable)	1m 43s
test (stable, --all-features)	2m 29s
test (beta, --no-default-features)	1m 40s
test (beta)	1m 41s
test (beta, --all-features)	2m 33s
test (nightly, --no-default-features)	1m 31s
test (nightly)	1m 31s
test (nightly, --all-features)	2m 15s
test (1.65, --no-default-features)	40s
test (1.65)	39s
Accept	0s
	20m 50s

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