rand::distributions -> distr; split uniform module

[dhardy]

• Added a CHANGELOG.md entry

Summary

This includes the contents of and will need a rebase after #1469, so the first real commit here is "Cfg-gate whole rand::seq::index module".

Implements #1381.

Motivation

I have long found the uniform.rs module hard to navigate, hence the split there.

Putting the contents in the existing float/int/other modules would need some quirky imports to keep the uniform public module intact (I also considered making this module private and directly re-exporting the contents, but it seems to serve a purpose, especially for documentation and to house the implementing types like UniformInt). Using new uniform/float.rs etc. modules results in a soft name-clash; the included path renames look better in my editor at least, but I can go the more orthodox approach if preferred.