Data/Tree: Add `Traversable` instance.

[BoltonBailey]

The Tree type defined in Mathlib/Data/Tree.lean should have a Traversable instance. This should be added to the file and the associated TODO removed.

[Komyyy]

@BoltonBailey Last year, I ported the LawfulTraversable instance deriver which derives Traversable attendantly in Mathlib.Tactic.DeriveTraversable.
You can see the example in test.Traversable.
This instance deriver should help you. 😄

[edegeltje]

@Komyyy from the looks of it, it seems your deriving handler creates functions that are not as polymorphic as they could be... for example, right now #check Tree.map gives Tree.map.{u, u_1} {α : Type u} {β : Type u_1} (f : α → β) : Tree α → Tree β, while using your deriving handler gives Tree.map.{u} {α β : Type u} (f : α → β) : Tree α → Tree β. As such, for now i will write the instance manually.