zcash_keys: Enable `no_std` usage. #6296

Job	Run time
Test no-Orchard on Linux	4m 46s
Test Orchard on Linux	6m 43s
Test NU7 on Linux	6m 41s
Build target wasm32-wasi	1m 4s
Latest build on ubuntu-latest	2m 16s
Check ZFuture build on Linux	37s
Latest build on windows-latest	5m 13s
Test no-Orchard on macOS	6m 46s
Check ZFuture build on macOS	42s
Latest build on macOS-latest	3m 0s
Test Orchard on macOS	11m 9s
Check ZFuture build on Windows	5m 38s
Slow Test Orchard on Linux	4m 28s
Test no-Orchard on Windows	8m 29s
Slow Test NU7 on Linux	4m 33s
Test Orchard on Windows	14m 2s
Test NU7 on Windows	14m 23s
Bitrot check	1m 41s
Clippy (MSRV)	1m 49s
Clippy (beta)	15s
Code coverage	1h 34m 37s
Intra-doc links	2m 17s
Rustfmt	29s
protobuf consistency	1m 17s
UUID validity	6s
Required status checks have passed	1s
	3h 23m 2s

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