Allow constructing recursive MapDiffer that uses itself

[JeroenDeDauw]

Fixes #98

[JeroenDeDauw]

@BoShurik I made a follow up, now it should be ready for merging.

Really a hacky solution though. Making this follow up made me realize how strange it is that the Differ return an array of DiffOp rather than a Diff. I don't see any reason for that. If they returned a Diff, we'd not have the problem in MapDiffer, that it needs to know if the inner differ is associative or not.

I'm thinking of just creating a version 4.0 that

• Ditches Diff in favor of new ListDiff and MapDiff that both extend a common Diff interface
• ListDiffer interface that returns ListDiff
• MapDiffer interface that returns MapDiff
• Add isAssociative to DiffOp, maybe removing isAtomic
  If that is done, then perhaps adding the MapDifferInterface now is a bit silly.

So I am not comfortable with merging this without review from someone else that thinks this is OK.

[JeroenDeDauw]

Another non-big-changes approach that avoids introducing MapDifferInterface is to add a new constructor argument $recursiveMapDiffs that would then replace the instanceof check in the current PR.