curve: use `subtle::Choice` for constant-time fixes #374

Job	Run time
Check that benchmarks compile	2m 48s
Test 32/64 bit stable (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	2m 50s
Test 32/64 bit stable (x86_64-unknown-linux-gnu)	2m 9s
Build serial on no_std target (thumbv7em-none-eabi) (curve25519-dalek)	19s
Check that clippy is happy	23s
Build serial on no_std target (thumbv7em-none-eabi) (ed25519-dalek)	25s
Build serial on no_std target (thumbv7em-none-eabi) (x25519-dalek)	18s
Check docs	28s
Check formatting	4s
Test Nightly	1m 16s
	11m 0s

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