Replace class iterator and const_iterator by using a single template class to reduce code.

🔀 fix issue #380: Signed integer overflow check

See https://travis-ci.org/nlohmann/json/jobs/183684093 for a failing
build.

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.