GHC.Ix.indexError no longer exported

As of base 4.14, the Ix typeclass's definition has been moved from GHC.Arr to GHC.Ix. However, the move didn't preserve the old GHC.Arr API; in particular, the indexError function is no longer exported.

indexError is useful when writing your own Ix instances, so it should be available to third-party code that creates new instances.

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