Support zeroizing for `SubgroupPoint` (#682) #336

Job	Run time
Build fiat on no_std target (thumbv7em-none-eabi) (curve25519-dalek)	24s
Test Build Script	36s
Current MSRV is 1.60.0	1m 59s
Test fiat backend (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	1m 20s
Test serial backend (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	1m 18s
Test simd backend (nightly)	49s
Test fiat backend (x86_64-unknown-linux-gnu)	56s
Test serial backend (x86_64-unknown-linux-gnu)	52s
Test simd backend (stable)	55s
	9m 9s

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