feat: expose rng trait hidden

[saiintbrisson]

This change exposes the rand::Rng as a hidden re-export, allowing users to derive with Dummy
without having to add the rand crate to their dependencies.

Currently, simple uses like deriving the Dummy macro requires one to add the rand crate to their
dependency tree, for it to not even be used directly in the code.

[saiintbrisson]

I think this one requires a dummy release as well if I'm not mistaken.