Fix basic_iterator for output cursors that don't provide post_increment().

[tahonermann]

• Added a post-increment operator overload to basic_iterator that returns a
  self reference for non-input cursors that don't provide post_increment().

• Added a test to validate that output iterators that hold state are correctly
  updated for post-increment dereference assignment.

• Updated a static_assert for ostream_iterator to reflect that post-increment
  now returns a self reference. This is consistent with the implementation of
  ostream_iterator in libstdc++.