curve: use `subtle::Choice` for constant-time fixes #249

Job	Run time
Build fiat on no_std target (thumbv7em-none-eabi) (curve25519-dalek)	21s
Test serial backend (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	1m 11s
Test Build Script	35s
Test serial backend (x86_64-unknown-linux-gnu)	50s
Current MSRV is 1.60.0	1m 55s
Test simd backend (nightly)	55s
Test fiat backend (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	1m 15s
Test simd backend (stable)	51s
Test fiat backend (x86_64-unknown-linux-gnu)	50s
	8m 43s

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