This adds the following table features:
- row span and column span
- footer row
- individual horizontal cell alignment
[API change] T.P.Writers.Shared: Add functions `allRowsEmpty` and
`tableBodiesToRows` from the RST writer for reuse in other writers.
Also fix hlint warning about `unzip` from `NonEmpty`:
The exported polymorphic function will become
monomorphic in the future. Restrict the `NonEmpty`
import to use the Prelude `unzip` function.
