Add Writers.Tables helper functions and types, add tests for those
The Writers.Tables module contains an AnnTable type that is a pandoc
Table with added inferred information that should be enough for
writers (in particular the HTML writer) to operate on without having
to lay out the table themselves.
The toAnnTable and fromAnnTable functions in that module convert
between AnnTable and Table. In addition to producing an AnnTable with
coherent and well-formed annotations, the toAnnTable function also
normalizes its input Table like the table builder does.
Various tests ensure that toAnnTable normalizes tables exactly like
the table builder, and that its annotations are coherent.
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