Changelog for v0.6.6 is incomplete

[sjakobi]

See https://hackage.haskell.org/package/containers-0.6.6/changelog.

Some changes that aren't mentioned:

• Fix #814: add argSet and fromArgSet #817
• Change IntMap.lookup and add new IntMap.query function #800
• Implement 'intersections', and add an 'Intersection' newtype along with a 'Semigroup' instance #756