Move rand_distr to new repository

[dhardy]

Motivation

rand_distr depends on rand, but nothing else here depends on rand_distr. Although rand_distr must reflect changes in rand::distributions (or rand::distr - #1381), it is a mistake to co-version these code-bases: a few breaking changes to rand are the only thing preventing us from making a new release of rand_distr (overdue).

There is some motivation for co-versioning the other crates in this repository: rand_core, rand_chacha, rand_pcg and rand all have inter-dependencies. The same is not true for rand_distr.

Further, it is usually very clear whether an issue relates to rand_distr or other parts of rand.

Plan

There are two obvious approaches:

1. Copy rand_distr code to a new repo. Adjust the rand dependency to the last released version, reverting a few breaking changes (Uniform::new returning a Result, ...?). Plan the next release.
2. Wait until the next stable rand release (slightly less work).