Currently we have a few problems with our feature gating, attempt to
audit all feature gating and fix it by doing:
1. Do not enable features on optional dependencies (`rand` and
`bitcoin-hashes`) in dev-dependencies, doing so hides broken feature
gating in unit tests.
2. Do not use unnecessary feature combinations when one feature enables
another e.g. `any(feature = "std", feature = "alloc")`.
3. Enable "std" from "rand-std" and "bitcoin-std" (and fix features
gating as for point 2).
4. Clean up code around `rand::thread_rng`, this is part of this patch
because `thread_rng` requires the "rand-std" feature.
5. Clean up CI test script to test each feature individually now that
"rand-std" and "bitcoin-hashes-std" enable "std".
Tobin C. Harding
Elichai Turkel