simple-linked-list: add canonical data

[loziniak]

references #568 and #587

[angelikatyborska]

I am a bit surprised by the property names you're suggesting in this PR, fromArrayAndBack and convertReverseConvertBack? Did you base that on some existing implementation of this exercise?

I'm looking at the implementations from some of the most popular tracks:

• JavaScript
• Python
• Ruby

They all include push and reverse, two of them also pop. Should those properties be part of the problem specification?

I think the challenge in writing problem specs for this exercise is to choose some representation of linked lists that will be different from a JSON array, to be able to write specifications for properties something like fromArray and toArray. I'm looking at the problem specification of linked-list (https://github.com/exercism/problem-specifications/blob/main/exercises/linked-list/canonical-data.json) and there the problem is worked around by only specifying chains of operations that result in an empty list 🤔.

I'm not really sure what should happen in problem specs for this exercise but I wanted to kick off a discussion so that this PR doesn't go ignored. @exercism/reviewers anyone would like to chime in?

[loziniak]

I suggested these methods as an indirect way of testing methods, that are subject of the exercise:

"lets create a singly linked list to contain the range (1..10), and provide functions to reverse a linked list and convert to and from arrays." (problem description)

To test function that creates a list from an array, we should encode somehow a linked list in JSON to check it by expected field in canonical data, same with reverse function, there a list should also be an input. I had no idea how to do this.

fromArrayAndBack is simply a superposition of fromArray and then toArray. And convertReverseConvertBack is fromArray → reverse → toArray. You can see the idea in my example solution in Red track. Now I think perhaps it's a good idea to add a track-specific notice about this anyway.