ENH: avoid std::distance for std::initializer_list

- std::initializer_list has its own size() method, so no need to use
  std::distance.

STYLE/BUG: use separate iterator de-reference and increment in List

- avoids unnecessary copying of iterators, and avoids any potentially
  odd behaviour with the combination with incrementing.

ENH: support construct from iterator pair for DynamicList, SortableList