Add SnakList constructor

[thiemowmde]

This introduces a constructor to the SnakList class that does not call the parent constructor. This is done on purpose to decrease the inheritance and possibly get rid of the ArrayObject dependency some day.

This also drops the possibility to call the constructor with null and adds the possibility to call it with variable length arguments (as some similar constructors and add methods already do).

This is split from #416.

[JeroenDeDauw]

There are comments by @snaterlicious and me on #376 questioning the merrit of this change.

[thiemowmde]

Note that the comment above is not applicable any more because I removed the "variable number of arguments" feature entirely.