Re-org with distr::slice, distr::weighted modules

[dhardy]

• Added a CHANGELOG.md entry

Summary

Re-organise / rename a few things in rand::distr and rand_distr:

• Move Slice -> slice::Choose, EmptySlice -> slice::Empty
• Rename trait DistString -> SampleString
• Rename DistIter -> Iter, DistMap -> Map
• Move {Weight, WeightError, WeightedIndex} -> weighted::{Weight, Error, WeightedIndex}
• Move weighted_alias::{AliasableWeight, WeightedAliasIndex} -> weighted::{..}
• Move weighted_tree::WeightedTreeIndex -> weighted::WeightedTreeIndex

Motivation

Some things like Slice and DistString weren't that obvious from the name. Now I find the purpose clearer from the path.

For rand_distr, all weighted distributions are now under a single module which is better for doc and shared stuff like Weight. (Possibly we should go further and move them all to a new crate? But maybe not since rand::seq::IndexedRandom also supports weighted sampling.)

Possibly more re-organisation of rand_distr is justified. The "flat organisation" system works well enough though.

[benjamin-lieser]

It looks fine to me (minus the two comments on the CI) and I think it makes sense to organize it in this way.