Add non-const overloads of the * and -> for filter_iteratort

This adds support for moving from the values accessible through the
result of `ranget.filter`.

Author: thomasspriggs

src/util/range.h

@@ -142,6 +142,16 @@ class filter_iteratort
     return tmp;
   }
 
+  value_type &operator*()
+  {
+    return *underlying;
+  }
+
+  value_type *operator->()
+  {
+    return &(*underlying);
+  }
+
   const value_type &operator*() const
   {
     return *underlying;