ocaml: phase 5.1 btree.ml baseline (13/13 pass)

Polymorphic binary search tree with insert + in-order traversal.
Exercises parametric ADT (type 'a tree = Leaf | Node of 'a * 'a tree
* 'a tree), recursive match, List.append, List.fold_left.

lib/ocaml/baseline/expected.json

@@ -1,4 +1,5 @@
 {
+  "btree.ml": 39,
   "calc.ml": 13,
   "closures.ml": 315,
   "exception_handle.ml": 4,