Rng::gen_index; no usize for Standard

[dhardy]

• Added a CHANGELOG.md entry

Summary

Follows #1470, hence the first commit of this PR is "cfg-gate IndexVec::USize".

Motivation

Solve #1399 by removing support for sampling usize, isize through the usual methods.

Details

This is potentially the most drastic and problematic approach, but seems to work fine at least for rand's tests. Specifically, it removes:

• Standard distribution support for usize, isize and non-zero and SIMD variants
• Uniform distribution support for the same types
• Support for Fill-ing [usize], [isize]
• Adds Rng::gen_index
• WeightedAliasIndex no longer supports isize or usize weights

This is meant for discussion. Potentially we could also use in a release with the option to add back support for the above types (this would be non-breaking).

fn Rng::gen_index supports various range types over usize using a private trait bound. Unorthodox and it's possible that the lack of ability for downstream crates to write generics over this would be an issue, but that is really not an intended feature. (We could also simplify this down to just RangeTo; that's all that rand uses anyway.)

We might also want to expose UniformUsize (used in rand::distr::slice), probably under rand::distr::uniform (alongside UniformInt, possibly without implementing SampleUniform though that is an option).

[dhardy]

I added UniformUsize (so Uniform supports usize). This uses 32-bit sampling where possible, which seems reasonable since in practice the high value will almost always be under u32::MAX. I haven't benchmarked against prior performance because I'm more interested in portability for the case where this is used.

isize for Uniform is omitted because I don't see any use-case (if there is, what is the expected behaviour?).

Fill and Standard still omit isize and usize support because... I can't think of a single reasonable usage, and further these are inherently non-portable.

[vks]

Do we still need Rng::gen_index if Rng::gen_range supports the same use case? (We would have to implement SampleUniform for UniformUsize.)

[dhardy]

It turns out that due to the bounds on SampleBorrow, any implementor of SampleUniform must also implement SampleUniform.

Rng::gen_index supports ..end and ..=high which gen_range does not, but otherwise... no, we don't need gen_index. Perhaps removing that is the better option (less disruptive).

[dhardy]

#1487 was merged instead