Use defensive `Data.Foldable` import

With this `import`-style containers will compile warning free
with existing GHC versions as well as GHC HEAD (in its current form)

This change is also needed because `Data.Foldable` is planned to export
`null` and `length` which will otherwise clash with `Data.Sequence`
Merge pull request #54 from hvr/pr-foldable

Use defensive `Data.Foldable` import