ed25519-dalek: hide secret in `SigningKey`'s `Debug` impl #135

Job	Run time
Build fiat on no_std target (thumbv7em-none-eabi) (curve25519-dalek)	37s
Test Build Script	55s
Test fiat backend (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	2m 49s
Test serial backend (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	2m 20s
Test fiat backend (x86_64-unknown-linux-gnu)	1m 31s
Test serial backend (x86_64-unknown-linux-gnu)	1m 27s
Current MSRV is 1.60.0	6m 28s
Test simd backend (nightly)	1m 41s
Test simd backend (stable)	2m 13s
	20m 1s

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