Add `Choice#index` and `Choice#<=>`

[DanielVartanov]

Describe the change

It adds #index and #<=> methods to Choice which makes them sortable inside Choices.

Why are we doing this?

In PR https://github.com/piotrmurach/tty-prompt/pull/141 Piotr suggested

I wonder if we could extend the Choice with index attribute or similar and make it comparable. This would simplify and speed up sorting: @selected.sort!

Benefits

@selected.sort_by! { |choice| choices.index(choice) } can be replaced with @selected.sort!

Drawbacks

• Breaks a promise of immutability of instances of Choice as described in public API
• Breaks backward compatibility by pulling Choices#choices (an Array) from the public API to avoid direct addition without setting an index.
• Breaks a convention of Comparable mixin which requires coherence of <=> and == operators.
• Since instances of Choice can be created outside Choices, the former can theoretically be added to more than one Choices which requires additional checks and considerations.
• Choice#index is left mutable from the outside (though not in public API)

Requirements

Put an X between brackets on each line if you have done the item: [X] Tests written & passing locally? [X] Code style checked? [X] Rebased with master branch? [X] Documentation updated?

[coveralls]

Coverage increased (+0.01%) to 97.352% when pulling 03544ebe5b7e70f9df06aed67b974f23c4218a1a on DanielVartanov:make-choices-sortable into b330a32eb0d509b9d515d59819b9745e0cea0b42 on piotrmurach:master.

[DanielVartanov]

Tests failed for older versions of Ruby (<2.3). @piotrmurach if you don't mind I'll fix them only if you choose this path, no point of polishing it if the other path is chosen

[DanielVartanov]

Great, I thought the same the very moment I realised I had to remove immutability :+1: Anyway it was good to explore this path, as you see there are something which are better be cherry-picked

[piotrmurach]

Thank you for exploring this path, helped me see the impact of this idea pretty clearly.