Allow SpendValidatingKey to be constructed from bytes #3196

Job	Run time
Bitrot check	3m 3s
Book tests	16s
Build target wasm32-wasi	2m 32s
Latest build on ubuntu-latest	54s
Intra-doc links	3m 15s
Latest build on windows-latest	1m 50s
Rustfmt	17s
Code coverage	9m 5s
Test on ubuntu-latest	9m 1s
Test on windows-latest	10m 1s
Latest build on macOS-latest	48s
Test on macOS-latest	8m 0s
	49m 2s

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