add Tree.map_branches

[pveber]

let map_branches t ~f =
  let root_data = function
    | Tree.Leaf l -> `Leaf l
    | Tree.Node n -> `Node n.data
  in
  let rec traverse_node = function
    | Tree.Leaf _ as l -> l
    | Node n ->
      Tree.node n.data (List1.map n.branches ~f:(traverse_branch n.data))
  and traverse_branch parent_data (Branch bi) =
    Tree.branch
      (f parent_data bi.data (root_data bi.tip))
      (traverse_node bi.tip)
  in
  traverse_node t