forest insert also handles move_constructible only types.

Insert now moves incoming object into node.
Insert now takes incoming object by value.
Chose take by value and move approach to limit code duplication and diff noise.

adobe/forest.hpp

@@ -655,8 +655,8 @@ class forest {
     iterator erase(const iterator& position);
     iterator erase(const iterator& first, const iterator& last);
 
-    iterator insert(const iterator& position, const T& x) {
-        iterator result(new node_t(x), true);
+    iterator insert(const iterator& position, T x) {
+        iterator result(new node_t(std::move(x)), true);
 
         if (size_valid())
             ++size_m;