Started building [[todo/import_tree]] (in the `importtree` branch). So far
the content identifier storage in the git-annex branch is done. Since the
API tells me it will need to both map from a key to content identifiers,
and from content identifier to the key, I also added a sqlite database to
handle the latter.

While implementing that, I happened to notice a bug in storage of metadata
that contains newlines; [[internals]] said that would be base64'd, but it
was not. That bug turns out to have been introduced by the ByteString
conversion in January, and it's the second bug caused by that conversion.
The other one broke git-annex on Windows, which was fixed by a release
yesterday.
