Add Array[Symbol.species] to symbol

[ddbeck]

I'll admit it: this is a bit of a hack.

Like other species keys, this key properly belongs in the array feature. But it we can't put it there just yet.

Because we added the array features early, it's broken down much more finely than other JavaScript collections (and lot of it ought to be combined, when we've dealt with #91). That means this key doesn't fit into very neatly into array or any of the other existing array features.

But it also doesn't make sense as a standalone feature. It's really hard to describe an iffy contrivance as a "feature." I'm not even sure what the pitch is, exactly ("You can make a subclass of Array pretend to be an array"?). It's too weird.

So here I propose parking the key with the symbol feature and cleaning this up more thoroughly later.

[Elchi3]

Agree to merge this into a larger array feature at a later stage and happy to hack it into symbol for now.