Thread rng

[dhardy]

• Added a CHANGELOG.md entry

Summary

Add the thread_rng feature flag

Motivation

• Symmetry with small_rng, std_rng, os_rng feature flags
• Cleaner feature tags in docs (having std and std_rng and os_rng on rng() etc. is not pretty)
• The possibility of Let rand::rng() use OsRng unless thread_rng is enabled? #1545 as a non-breaking change (though it likely won't happen)

[newpavlov]

Looks good. My only concern is the features circularity (std -> thread_rng -> std). I never used them in practice, so I don't know whether they may cause issues or not.

[dhardy]

My only concern is the features circularity (std -> thread_rng -> std).

But std doesn't require thread_rng? default does.

[newpavlov]

Ah, true. I wasn't looking careful enough.