WeightedIndex distribution naming

[dhardy]

As noted in the original PR and here, we're not entirely happy with the path/name of weighted index distributions:

use rand::distributions::weighted::WeightedIndex;
// or:
use rand::distributions::weighted::alias_method::WeightedIndex;

These paths are long, slightly repetitive ("weighted"), and one of the paths includes the algorithm name which we usually avoid. Also, Index is potentially unnecessary (this is designed for indexing weighted arrays, but is essentially just a biased finite integer distribution).

This needs review before 0.7.

[dhardy]

An alternative may be to move this to rand_distr or a new crate. Rationale:

• it is not very commonly used (154 hits for rand + weightedindex on GH with lang:rust vs 87920 for just rand)
• aside from Bernoulli, this is the only non-linear distribution included in Rand

[vks]

Moving it to rand_distr is problematic, because rand::seq needs it, and we don't want rand to depend on rand_distr. This would probably require moving rand::seq to its own crate.

[dhardy]

True, there are some uses of SliceIterator::choose_weighted, but not many (64 total hits, many of them just code copies).

We could instead use another extension trait, but at only 738 lines for both weighted impls it's barely worth it.

[newpavlov]

Closed in #1008.