Add `flattenSCC1 :: SCC a -> NonEmpty a`

[andreasabel]

This gives a more precise type to the existing flattenSCC :: SCC a -> [a]. Closes #985.

[Bodigrim]

This probably needs a changelog entry and @since pragma, but I don't know what's the policy here and which version number is supposed to be released next.

[andreasabel]

This probably needs a changelog entry and @since pragma, but I don't know what's the policy here and which version number is supposed to be released next.

I can add that here once the decision has been taken to merge this PR!

[treeowl]

Let's do it.

[andreasabel]

Ok, I added a since for 0.7.1.

I was a bit surprised to see master at 0.6.8 when it already contained the changes for 0.7. According to the changelog, master also contains breaking changes which would require the next master release to be 0.8.