Add VerifyingKey::to_edwards #271

Job	Run time
Check docs	34s
Check docs	38s
Build serial on no_std target (thumbv7em-none-eabi) (curve25519-dalek)	22s
Build serial on no_std target (thumbv7em-none-eabi) (curve25519-dalek)	22s
Check that benchmarks compile	2m 52s
Check that benchmarks compile	2m 53s
Check that clippy is happy	25s
Check that clippy is happy	23s
Build serial on no_std target (thumbv7em-none-eabi) (ed25519-dalek)	36s
Build serial on no_std target (thumbv7em-none-eabi) (ed25519-dalek)	35s
Build serial on no_std target (thumbv7em-none-eabi) (x25519-dalek)	30s
Build serial on no_std target (thumbv7em-none-eabi) (x25519-dalek)	27s
Check formatting	14s
Check formatting	12s
Test Nightly	33s
Test Nightly	33s
Test 32/64 bit stable (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	2m 35s
Test 32/64 bit stable (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	2m 40s
Test 32/64 bit stable (x86_64-unknown-linux-gnu)	2m 8s
Test 32/64 bit stable (x86_64-unknown-linux-gnu)	2m 52s
	22m 24s

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