Use static_cast on digit.

Instead of checking something like `x * y + z > max` where `x * y` can
overflow, check for `x > (max - z) / y` instead.

Replaced class iterator and class const_iterator with aliases of template <typename U> iter_impl. iter_impl has operator const_iterator to create an const_iterator from an iterator.

Snippet from http://stackoverflow.com/a/41031865/266378