Remove small_rng feature?

[dhardy]

Summary

Remove small_rng feature, thus enabling SmallRng by default and in all builds.

Motivation

Less complexity. Removes a trip-hazard for new users: #1225

If I remember correctly, the motivation for feature-gating adding small_rng was to remove a dependency from builds which don't use it. Later, we decided to add std_rng for the same reason (even though ThreadRng requires it).

We now define the two possible implementations of SmallRng in crate (around 250 LoC). If we changed the impl again, we'd very likely continue to do so (it would be unlikely to use more code).

The additional (small) cost is that SmallRng must be compiled in all builds; linking should remove unused code thus not affecting binary size.

[newpavlov]

The trip-hazard can also be resolved by making small_rng a default feature.

Assuming SmallRng implementation will continue to be part of the rand crate, personally, I am fine with removing the feature.