Let rand::rng() use OsRng unless thread_rng is enabled?

[dhardy]

ThreadRng adds some thread-local memory. In case not all users want this, we could add a thread_rng feature flag (probably default-enabled).

Where the thread_rng feature is not enabled, rand::rngs::ThreadRng would not be available. rand::rng() would instead use OsRng (assuming that getrandom is available), thus thread_rng would mostly be a performance boost (at the cost of extra code and thread-local memory).

[newpavlov]

Personally, I would be somewhat surprised by such behavior, so I lean slightly against it. Though, it's not a strong opinion, so I would be fine with such change.