[cleanup] Hide FlatVectorBackend in favour of flatVectorView()

* Hide the old implementation class FlatVectorBackend in
  namespace Impl::. It was never documented anyway.
* Add a new class FlatVectorView. In contrast to the old one,
  this behaves like a (view-) value providing a very slim
  flat-vector-like interface instead of odd static methods.
* To be flexible FlatVectorView is also hidden in Impl::
  Instead we only expose factory methods:
* Add factory methods flatVectorView() creating the view
  objects. When passing a const/mutable l-value reference
  the returned object will store a pointer to the underlying
  container. If the passed container is an r-value reference
  the object will be stored by value inside of the view.
  This is necessary to support proxy objects.