Add drop functions for ranget

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

We have two versions of this, both return a new range, but one is used
when the range on which the method is called can be moved.

Author: romainbrenguier

src/util/range.h

@@ -341,6 +341,26 @@ struct ranget final
     return begin_value == end_value;
   }
 
+  /// Drop the first \p count elements.
+  /// If the range has fewer elements, drops everything.
+  /// Similar to \ref skip but modifies the range in-place.
+  /// \return true if the range still contains element after the drop
+  ranget<iteratort> drop(std::size_t count) &&
+  {
+    for(; count > 0 && begin_value != end_value; --count)
+      ++begin_value;
+    return ranget<iteratort>{std::move(begin_value), std::move(end_value)};
+  }
+
+  /// Return an new range containing the same elements except for the first
+  /// \p count elements.
+  /// If the range has fewer elements, returns an empty range.
+  /// Similar to \ref drop but returns a new range object.
+  ranget<iteratort> drop(std::size_t count) const &
+  {
+    return ranget<iteratort>{begin(), end()}.drop(count);
+  }
+
   iteratort begin() const
   {
     return begin_value;