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Previously, `Array`s were simply converted to lists, and the list methods used. That works acceptably well for `foldr` and `foldr1`, but not so sensibly for most other things. Left folds ended up "twisted" the way they are for lists, leading to surprising performance characteristics. Moreover, this implements `length` and `null` so they check the array size directly. Finally, a test is added to the testsuite ensuring the overridden `Foldable` methods agree with their expected default semantics. Addresses #9763 Reviewed By: hvr, austin Differential Revision: https://phabricator.haskell.org/D459
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