DemuxArena should use the same arena implementation as the source

[recht]

If you marshal a SingleSegment and unmarshal it again, you get a MultiSegment. If you then start writing to that then you get several segments instead of using a single segment.

I can see why this can be tricky since the sender and the receiver might be different systems, but it is somwhat surprising. Especially given #97.

[zombiezen]

So when I was evaluating this (I did make a conscious decision to do this), my rationale was getting a different arena allocator based on the number of segments seemed surprising, and that if this control was needed, then the application code could do it. Admittedly, I don't expose the header parsing yet, but I feel much better about the current parse function than previous iterations.