Add reverse iterators to `Node`/`TNode` (#6825)

Add reverse iterators to `Node`/`TNode` (#6825)

This feature is useful for example for succinctly inserting children of
a node in the reverse order. To make this work, the commit fixes the
declaration of `reference` in the `NodeValue::iterator`. The
`std::reverse_iterator` adapter expects the `reference` type to match
the return type of `operator*` in the corresponding iterator (as
mandated by the standard). Despite its name, `reference` does not have
to be a C++ reference. Note that we cannot actually make it a C++
reference because `NodeValue::iterator` wraps the `NodeValue*` in a
`Node`/`TNode` in `operator*`.