Issue Description:

IndexPath has poor performance when bridging from objc and bridges too often for compare.

public func compare(_ other: IndexPath) -> ComparisonResult incurs two bridges out to objc which would potentially allocate two NSIndexSets; this is not needed and can be done strictly in swift; which avoids allocations to compare.

fileprivate init(nsIndexPath: ReferenceType) calls malloc to fetch indices from the NSIndexSet reference. This can be done via Array which in common cases is faster.

This applies to the Darwin overlay.