zcash_client_backend: Implement note management via change splitting. #5947

Job	Run time
Build target wasm32-wasi	56s
Latest build on ubuntu-latest	2m 6s
Test on Linux	9m 38s
Latest build on windows-latest	3m 58s
Test Orchard on Linux	11m 5s
Latest build on macOS-latest	2m 20s
Bitrot check	1m 48s
Test NU6 on Linux	9m 31s
Test on macOS	10m 27s
Clippy (MSRV)	1m 33s
Test Orchard on macOS	13m 6s
Check ZFuture build on Linux	23s
Test on Windows	16m 9s
Check ZFuture build on macOS	36s
Test Orchard on Windows	20m 10s
Check ZFuture build on Windows	2m 55s
Clippy (beta)	13s
Test NU6 on Windows	16m 45s
Code coverage	19m 28s
Intra-doc links	1m 58s
Rustfmt	13s
protobuf consistency	1m 42s
UUID validity	4s
	2h 27m 4s

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