[WIP] Do not build SnakList on top of ArrayObject

[thiemowmde]

The main reason to touch this file again is the awkward complexity it had before. Note that the data structure was build in a way that it kept track of two keys for each element. One key is the snak hash, the other one whatever you provided via the constructor.

WIP mostly because of the Iterator methods (rewind, current, and so on). These expose the internal state of the $this->snaks array. I'm not sure if this is robust enough. The array pointer might change when using other SnakList methods.

[JeroenDeDauw]

The index change is nice though technically a breaking change. Since making a major release is currently very expensive, it might be better to avoid this one. We could also just present it is not a breaking change... though that is a bit scary. Something could be relying on the old behavior.

As to the Iterator stuff: you can implement IteratorAggregate instead. And in its getIterator method, just do

yield from $this->snaks;