Update to orchard_auth_digest computation #1744

Job	Run time
Bitrot check	2m 47s
Book tests	12s
Latest build on ubuntu-latest	1m 9s
Build target wasm32-wasi	2m 5s
Latest build on windows-latest	2m 47s
Code coverage	19m 24s
Latest build on macOS-latest	54s
Intra-doc links	3m 3s
Rustfmt	12s
Test on ubuntu-latest	33m 53s
Test on windows-latest	34m 28s
Test on macOS-latest	28m 8s
	2h 9m 2s

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