B+TREE: Add _pop function.

m-bptree.h

@@ -530,5 +530,16 @@
     }                                                                   \
   }                                                                     \
                                                                         \
+  static inline bool M_C(name, _pop)(value_t *ptr, tree_t b, key_t const key) \
+  {                                                                     \
+    if (ptr != NULL) {                                                  \
+      value_t *ref = M_C(name, _get)(b, key);                           \
+      if (ref == NULL) {                                                \
+        return false;                                                   \
+      }                                                                 \
+      M_GET_SET value_oplist (*ptr, *ref);                              \
+    }                                                                   \
+    return M_C(name, _remove)(b, key);                                  \
+  }                                                                     \
 
 #endif