Allow SpendValidatingKey to be constructed from bytes #3201

Job	Run time
Book tests	15s
Latest build on ubuntu-latest	49s
Build target wasm32-wasi	2m 34s
Bitrot check	3m 10s
Latest build on windows-latest	2m 26s
Code coverage	8m 57s
Latest build on macOS-latest	56s
Intra-doc links	3m 19s
Rustfmt	15s
Test on ubuntu-latest	9m 7s
Test on windows-latest	10m 24s
Test on macOS-latest	7m 37s
	49m 49s

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