Users may still want to derederence a const iterator (note: not a const_iterator).
Also use the "reference" typedef to ensure there is only 1 source of information
For rule of 5/0, where no implementation is required, all 5 operations have been declared as defaulted. This is less likely to forget definitions for all 5 if required
- removed forwarding of copy to assignment (which was defaulted already) in favour of defaulted copy ctor
- added defaulted move assignment/ctor and destructor
