Turn a few existing folds into `Foldable`-methods (#9621)

Turn `toList`, `elem`, `sum`, `product`, `maximum`, and `minimum` into
`Foldable` methods. This helps avoiding regressions (and semantic
differences) while implementing #9586

Reviewed By: austin, dfeuer, ekmett

Differential Revision: https://phabricator.haskell.org/D231