ENH: add OListStream::swap(DynamicList<char>&)

- allows full recovery of allocated space, not just addressable range.

  This can be particularly useful for code patterns that repeatedly
  reuse the same buffer space. For example,

      DynamicList<char> buf(1024);

      // some loop
      {
          OListStream os(std::move(buf));
          os << ...

          os.swap(buf);
      }

   Can read back from this buffer as a second operation:

      {
          UIListStream is(buf);
          is >> ...
      }