WIP: Create an in-memory wallet backend. #5049

Job	Run time
Build target wasm32-wasi	1m 27s
Check ZFuture build on Linux	1m 39s
Bitrot check	1m 42s
Check ZFuture build on macOS	3m 21s
Latest build on ubuntu-latest	1m 24s
Check ZFuture build on Windows	3m 3s
Latest build on windows-latest	3m 50s
Clippy (MSRV)	1m 38s
Latest build on macOS-latest	2m 14s
Clippy (beta)	15s
Code coverage	16m 37s
Intra-doc links	2m 6s
Rustfmt	13s
protobuf consistency	1m 47s
Test on Linux	6m 30s
UUID validity	6s
Test Orchard on Linux	6m 35s
Test NU6 on Linux	6m 49s
Test on macOS	16m 33s
Test Orchard on macOS	11m 49s
Test on Windows	9m 59s
Test Orchard on Windows	10m 59s
Test NU6 on Windows	10m 45s
	2h 1m 21s

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