This example seems to indicate that it's not necessary to build
the full list:

    > Data.DList.head (pure 1 <> undefined)
    1

Currently, the `Foldable DList` instance relies on the default
implementation of `toList`. However, this default implementation is
somewhat indirect since it uses `foldr`. `toList` can be more
directly implemented as `toList = Data.DList.toList`, which avoids
the need for `foldr` entirely.

* From commercialhaskell/stackage#5271
* Closes #38