Answering questions in your midterm code

[Link to source file](https://github.com/baswi/cpppc/blob/master/midterm/code/Source.cpp#L104)

~~~c++
// begin cannot result in a constant expression ??? WHY ...?!?!?!?
auto begin() noexcept {
  return elem_iterator(_chunk, 0, _pos);
}
~~~

Because the constructor `elem_iterator` is not declared constexpr.
