feat: add DTreeMap/TreeMap/TreeSet iterators and slices (#10776)

This PR adds iterators and slices for `DTreeMap`/`TreeMap`/`TreeSet`
based on zippers and provides basic lemmas about them.

---------

Co-authored-by: Markus Himmel <[email protected]>

Author: wkrozowski

src/Std/Data/DTreeMap.lean

@@ -9,3 +9,5 @@ prelude
 public import Std.Data.DTreeMap.Basic
 public import Std.Data.DTreeMap.AdditionalOperations
 public import Std.Data.DTreeMap.Lemmas
+public import Std.Data.DTreeMap.Iterator
+public import Std.Data.DTreeMap.Slice

src/Std/Data/DTreeMap/Internal/Def.lean

@@ -78,4 +78,11 @@ def toListModel : Impl α β → List ((a : α) × β a)
 @[simp] theorem toListModel_inner {sz k v l r} :
   (.inner sz k v l r : Impl α β).toListModel = l.toListModel ++ ⟨k, v⟩ :: r.toListModel := rfl
 
+/--
+  Computes the size of the tree. Used for verification of iterators.
+-/
+def treeSize : Internal.Impl α β → Nat
+  | .leaf => 0
+  | .inner _ _ _ l r => 1 + l.treeSize + treeSize r
+
 end Std.DTreeMap.Internal.Impl