Add a drop function for ranget

This is useful for advancing the range without having to manipulate the
private `begin_value` field.

Author: romainbrenguier

src/util/range.h

@@ -341,6 +341,18 @@ struct ranget final
     return begin_value == end_value;
   }
 
+  /// Drop the first \p count elements.
+  /// If the range has fewer elements, drops everything.
+  void drop(std::size_t count)
+  {
+    std::size_t i = 0;
+    while(i < count && begin_value != end_value)
+    {
+      ++begin_value;
+      ++i;
+    }
+  }
+
   iteratort begin() const
   {
     return begin_value;