Move OsRng down to rand

[dhardy]

• Added a CHANGELOG.md entry

Summary

Moves OsRng from rand_core to rand.

Removes fns SeedableRng::from_os_rng and try_from_os_rng.

Motivation

This removes the dependency on getrandom, making rand_core dependency-free (unless serde is enabled) and allowing an easier path to stabilisation of rand_core. See #1666.

Details

Removed features std and os_rng from rand_core.

[dhardy]

This is certainly less ergonomic than before, but affected us very little.

Unknown: how much this impacts other users.

[vks]

This reverts a change we did a long time ago. Do you remember why we moved it from rand_os to rand_core in the first place?

[dhardy]

SeedableRng::from_os_rng requires OsRng. We want to keep SeedableRng in rand_core, so essentially we must choose between:

1. Do not offer a from_os_rng method (this PR). This works fine for us but there are clearly some users (GH search).
2. Status quo: rand_core depends on getrandom for from_os_rng.
3. Add an extension trait SeedableRngExt: SeedableRng in rand and implement from_os_rng there. We could also add from_thread_rng in that case.