Array operations

[gormster]

Pretty quickly realised that setProperty doesn't work to add new values to an array (you get, predictably, an "Index out of range" fatal error). So, we need append(_:forKey:) to add new values to the end of an array, and remove(at:forKey:) to remove & renumber the remaining elements. This should be executed using $push and $pop ideally, but if keys have both been added and removed then it may be necessary to use $set (I don't think there's a way to reorder an array in a single DB call).

I think possibly it might be better for the mutatedFields document to be the actual update document and just keep track of sets, unsets, pushes and pops as the Instance itself is mutated. Thoughts?

[Joannis]

Ah.. good one! I've honestly never mutated an array with Mainecoon. I was planning on making it the update document, but since I'm a bit loaded with work I didn't have the time to do this yet.