Add efficient linear combination for Montgomery forms #1767

Job	Run time
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 6s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 5s
build (1.73.0, thumbv7em-none-eabi)	45s
test (x86_64-unknown-linux-gnu, 1.73.0)	50s
careful	1m 18s
build-benchmarks	47s
build (1.73.0, wasm32-unknown-unknown)	42s
test (x86_64-unknown-linux-gnu, stable)	1m 3s
build (stable, thumbv7em-none-eabi)	51s
clippy	21s
build (stable, wasm32-unknown-unknown)	47s
cross (aarch64-unknown-linux-gnu, stable)	1m 51s
doc	30s
cross (powerpc-unknown-linux-gnu, stable)	1m 41s
minimal-versions	36s
miri (x86_64-unknown-linux-gnu)	42s
rustfmt	6s
miri (s390x-unknown-linux-gnu)	55s
	15m 56s

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