Rename WeightedError → WeightError; add IndexedRandom, IndexedMutRandom

[dhardy]

Fixes #1307. CC @LikeLakers2.

Renames WeightedError → WeightError, revises variants and re-exports from rand::seq.

Adjusts WeightError cases: WeightedTreeIndex (NaNs and try_sample @xmakro) and rand::seq::index::sample_weighted (replace panic; catch too-many-zeros).

Splits SliceRandom into multiple traits to solve #1307. I'm not wildly keen on the result but don't currently have a better idea.

[LikeLakers2]

I took a general look at 06fe3d8 (the commit where you split SliceRandom).

Unless I'm missing something (it's been a while since I touched rand, so I'm going off of what I put in #1307, plus my own intuition), I believe you've solved the issue I had. So thank you.

I'm not wildly keen on the result but don't currently have a better idea.

Personally, I'm a fan of this solution. Not only does it make it possible for non-std collections to implement random functionality, but it makes it as simple as impl-ing one method on one trait.

Still, if you come up with a better idea, don't be afraid to poke me and I'll tell you what I think.

[TheIronBorn]

Looks good to me

[dhardy]

Thanks @TheIronBorn