Add optimized Edwards addition and doubling algorithms #652

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	3m 29s
benches (stable)	58s
test (x86_64-unknown-linux-gnu, 1.85.0)	3m 37s
benches (1.85.0)	1m 8s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	4m 47s
cross (armv7-unknown-linux-gnueabihf, 1.85.0)	1m 28s
test (i686-unknown-linux-gnu, 1.85.0, sudo apt update && sudo apt install gcc-multilib)	4m 38s
cross (aarch64-unknown-linux-gnu, stable)	1m 24s
doc	24s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 22s
cross (powerpc-unknown-linux-gnu, stable)	1m 39s
cross (aarch64-unknown-linux-gnu, 1.85.0)	1m 20s
cross (powerpc-unknown-linux-gnu, 1.85.0)	1m 29s
build (1.85.0, thumbv7em-none-eabi)	54s
build (1.85.0, wasm32-unknown-unknown)	51s
build (stable, wasm32-unknown-unknown)	46s
build (stable, thumbv7em-none-eabi)	51s
	31m 5s

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