The current implementation of iterators had the following bugs:

- it was possible to assign `const_iterator` to `iterator`
- it was not possible to assign `iterator` to `const_iterator`
- operator* did not always return `value_type` e.g. for
  `const_iterator` this often returned `T&` instead of
  `const value_type&`
- increment operators always returned `iterators`. This caused
  e.g. `const_iterator + 1` to result in a `iterator` instead of
  a `const_iterator`
- comparision operators could not be used to compare
  `iterators` and `const_iterators` (`const_iterator == iterator`)