more work on seq_span

more work on seq_span
One problem that showed up was that if a SeqSpan is created on the stack,
the parent shared pointer wont work, so any subseqs will crash.

To fix that at the python layer I had to add a factory function that
proprely dynamically created a SeqSpan.

I tried to get the parent call to work but that kept returning a new object
which makes the python is function unhappy.