An Anchor has a location and an operation, which is either that it is
unchanged or that it has moved with a DeltaPos

    data Anchor = Anchor { anchor :: RealSrcSpan
                         , anchor_op :: AnchorOperation }

An EpaLocation also has either a location or a DeltaPos

    data EpaLocation = EpaSpan !RealSrcSpan !(Strict.Maybe BufSpan)
                     | EpaDelta !DeltaPos ![LEpaComment]

Now that we do not care about always having a location in the anchor,
we remove Anchor and replace it with EpaLocation

We do this with a type alias initially, to ease the transition.
The alias will be removed in time.

We also have helpers to reconstruct the AnchorOperation from an
EpaLocation. This is also temporary.

Updates Haddock submodule