curve: support zeroizing for `SubgroupPoint` #294

Job	Run time
Test Build Script	38s
Build fiat on no_std target (thumbv7em-none-eabi) (curve25519-dalek)	29s
Test serial backend (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	1m 21s
Test simd backend (nightly)	54s
Test serial backend (x86_64-unknown-linux-gnu)	59s
Test fiat backend (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	1m 10s
Test fiat backend (x86_64-unknown-linux-gnu)	1m 3s
Test simd backend (stable)	1m 3s
Current MSRV is 1.60.0	2m 33s
	10m 10s

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