feat: add Heap instances for TreeMap and ExtTreeMap

- Add Store and Heap instances for TreeMap K V cmp
- Add Store and Heap instances for ExtTreeMap K V cmp
- Add Store, Heap, RepFunStore, IsoFunStore instances for function types
- Prove getElem?_mergeWith' with full WF tracking through internal foldl

Uses standard library operations (alter, filterMap, mergeWith) directly
instead of custom wrappers. Key proof helpers track WF through foldl.

Closes #105

Author: alok

src/Iris/Std.lean

@@ -1,5 +1,7 @@
 import Iris.Std.Classes
 import Iris.Std.Expr
+import Iris.Std.Heap
+import Iris.Std.HeapInstances
 import Iris.Std.Nat
 import Iris.Std.Prod
 import Iris.Std.Qq