zcash_keys: Enable `no_std` usage. #6295

Job	Run time
Check ZFuture build on Linux	2m 9s
Test no-Orchard on macOS	9m 54s
Check ZFuture build on macOS	1m 57s
Slow Test Orchard on Linux	5m 28s
Check ZFuture build on Windows	5m 52s
Slow Test NU7 on Linux	4m 38s
Test Orchard on macOS	8m 57s
Test no-Orchard on Windows	8m 57s
Test Orchard on Windows	14m 16s
Bitrot check	1m 37s
Test NU7 on Windows	14m 7s
Latest build on ubuntu-latest	2m 3s
Latest build on windows-latest	4m 43s
Latest build on macOS-latest	2m 6s
Build target wasm32-wasi	55s
Clippy (MSRV)	1m 21s
Clippy (beta)	27s
Test no-Orchard on Linux	5m 28s
Code coverage	1h 38m 28s
Test Orchard on Linux	7m 19s
Test NU7 on Linux	7m 42s
Intra-doc links	2m 13s
Rustfmt	22s
protobuf consistency	1m 15s
UUID validity	6s
Required status checks have passed	1s
	3h 32m 21s

Provide feedback