Make operator* and operator-> const in all iterators.

C++ iterators basically model pointers. Hence a const pointer can still
be dereferenced to a mutable object (`T * const` *not* `const T*`). This
is not true for many iterators in itertools since they only have
non-`const` versions of `operator*`. This also violates C++ iterator
concepts, see [the github issue](ryanhaining#91).

This change basically replaces all non-`const` dereference operators
with `const` ones. This was straight forward in most cases:

- Some iterators own the data they return (note: that probably violates
  [`LegacyForwardIterator`](https://en.cppreference.com/w/cpp/named_req/ForwardIterator)).
  So those data fields were changed to `mutable`.
- `GroupBy` advances the group while dereferencing. This does not work
  when the iterator is constant. Moved the advancing to the constructor
  and increment operators. This is the only real behavior change, please
  review carefully.