Improve `Foldable` instance for `Array`

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