Remove the `forest_const_iterator` explicitly-declared copy constructor:

[fpelliccioni]

Remove the forest_const_iterator explicitly-declared copy constructor:

forest_const_iterator(const forest_const_iterator& x) : _node(x._node), _edge(x._edge) {}

Here:
https://github.com/stlab/libraries/blob/develop/stlab/forest.hpp#L634

Let the compiler do it.

[dabrahams]

See also #334

[dabrahams]

Contributions welcome!