Allow the use of List.foldr in top-scorers

[jiegillet]

Closes #56

I didn't change the analyzer comment

aggregateScorers is most easily written using List.foldl and updateGoalCountForPlayer.

because it's still true, but should I? If someone uses List.foldr but not updateGoalCountForPlayer, would they be confused and change both? It's not a huge deal I guess.

[ceddlyburge]

Looks good. I think it probably is worth updating the copy.

Maybe to something like this:
'aggregateScorers is most easily written using updateGoalCountForPlayer and List.foldl (list.foldr also works, but it a bit les ergonomic).'

[jiegillet]

Opened a PR for the copy update: exercism/website-copy#2288

I don't think it's fundamentally blocking this PR though, so I'll merge it now.