edwards: add `SubgroupPoint` traits #401

Job	Run time
Build serial on no_std target (thumbv7em-none-eabi) (curve25519-dalek)	23s
Check that benchmarks compile	2m 56s
Build serial on no_std target (thumbv7em-none-eabi) (ed25519-dalek)	35s
Check that clippy is happy	23s
Test Nightly	1m 16s
Test 32/64 bit stable (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	2m 33s
Build serial on no_std target (thumbv7em-none-eabi) (x25519-dalek)	26s
Test 32/64 bit stable (x86_64-unknown-linux-gnu)	2m 13s
Check docs	36s
Check formatting	10s
	11m 31s

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