update C++ headers

Author: 9il

include/mir/ndslice.h

@@ -253,6 +253,16 @@ struct mir_slice<Iterator, 1, mir_slice_kind::contiguous>
         return _iterator[index];
     }
 
+    auto&& backward(mir_size_t index)
+    {
+        return at(size() - 1 - index);
+    }
+
+    auto&& backward(mir_size_t index) const
+    {
+        return at(size() - 1 - index);
+    }
+
     auto&& operator[](mir_size_t index)
     {
         return at(index);

include/mir/series.h

@@ -71,6 +71,11 @@ struct mir_series
         return {_index[index], _data[index]};
     }
 
+    Observation backward(mir_size_t index) const noexcept
+    {
+        return {_index[size() - 1 - index], _data[size() - 1 - index]};
+    }
+
     Observation operator[](mir_size_t index) const noexcept
     {
         return {_index[index], _data[index]};