NoUnused.Variables: Detect unused mutually recursive functions

[jfmengels]

NoUnused.Variables doesn't notice that values and valuesFromList are used mutually but never used anywhere else (if that was the case).

type Tree value = Node value | Tree (List (Tree value))

values : Tree value -> List value
values tree =
	case tree of
		Node value -> [ value ]
        Tree list -> valuesFromList list

valuesFromList : List (Node value) -> List value
valuesFromList list =
  List.concatMap values list

We should improve the rule to make this kind of discovery and remove both functions.