rand_distr 0.3 tracker

[dhardy]

No real plans for this yet, but:

• (breaking): revise dirichlet::Error enum
• Migrate to num-traits Migrate rand_distr to num-traits for no_std support #987
• Update to rand v0.8

[vks]

The changes to beta sampling (#1000) modify the trait bounds, but I think it's not breaking, because we only drop bounds.

[vks]

This was done (see #1021), minus the update to rand 0.8, which was deferred to rand_distr 0.4.