Implement and test Foldable/Traversable instances

[MaybeJustJames]

Implement a Foldable instance for Graph.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[thomashoneyman]

Thanks! cc @colinwahl, as we were just working through this library together — do you have any thoughts?

[MaybeJustJames]

I have a followup with Traversable here. Would you like it in the same PR?

[JordanMartinez]

I'd say add it here.

[JordanMartinez]

Ideally, we would use 1 fold rather than 2 here (i.e. M.values folds once, then <$> folds a second time). But the single fold version...
foldl f z (foldl (\acc (Tuple k _) -> Array.snoc acc k) [] m)
... is likely less performant because of the usage of Array.snoc.

[MaybeJustJames]

Would something like this be better?

instance foldableGraph :: Foldable (Graph k) where
  foldl   f z (Graph m) = foldl   (\acc (Tuple k _) -> f acc k) z $ M.values m
  foldr   f z (Graph m) = foldr   (\(Tuple k _) acc -> f k acc) z $ M.values m
  foldMap f   (Graph m) = foldMap (f <<< fst) $ M.values m

[JordanMartinez]

Ah... probably.

[MaybeJustJames]

Done in 20b2649

[colinwahl]

@thomashoneyman Yep, looks fine to me!