BUG: MapIter leaked itself and arr and index objects

MapIterBind incorrectly behaved when failing, cleaning up most
but not everything. The callers have to decref the mapiter
object. Which basically means that MapIterBind does not even
have to decref the new fields (they are decrefed in the
mapiter dealloc anyway).

Note that trying to add tests checking the refcounting
failed, but the tests seem to fail at least in many cases
not because of the indexing.

Closes gh-3589

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OK, I just commented the failing tests (I think). It would be nice to track down, but correctly decrefing mapiter is better then nothing for 1.8. and I certainly do not have time to even try finding out where it might be going bad in the next weeks.