Previously the Foldable instance for NonEmpty used default
implementations for toList and length.

I assume that the existing implementations
(i.e. Data.List.NonEmpty.{toList,length}) are better than
the default ones, and frankly can't see a good reason why they
might be worse – but if they are, instead of this commit
we'd have to switch Data.List.NonEmpty.{toList,length}
to use Foldable.

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: int-index, thomie

Differential Revision: https://phabricator.haskell.org/D2882