Add `Ember.Subarray.prototype.toString`.

This is mostly helpful in debugging.
[BUGFIX release] `Ember.SubArray` operation composition fix.

In the case where `SubArray` would compose-left, the index was left unchanged,
and we would attempt to compose-right with bad assumptions about what operation
we were composing.
Merge pull request #3469 from hjdivad/fix-jsbin-ImEsOWu

Fix a bug in `SubArray` composition.