SI-7018 Fix memory leak in Attachments.

Makes NonEmptyAttachments a top level class so that
it doesn't accidentally accumulate history via the
$outer field.

No test is included because I think the fix is
self evident.
Update a checkfile from a recent fix.

The build is currently broken:

  https://scala-webapps.epfl.ch/jenkins/view/2.10.x/job/scala-nightly-main-2.10.x/
Merge pull request #1977 from retronym/ticket/7018

SI-7018 Fix memory leak in Attachments