This PR adds support for decidable equality of empty lists and empty
arrays. Decidable equality for lists and arrays is suitably modified so
that all diamonds are definitionally equal.
Following #9302, the strong condition of definitionally equal under
`with_reducible_and_instances` is tested. This also moves some of the
comments added in #9302 out of docstrings.
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Co-authored-by: Aaron Liu <aaronliu2008@outlook.com>
Co-authored-by: Eric Wieser <wieser.eric@gmail.com>
