[API change] Text.Pandoc.Parsing: New functions `tableWithSpans`,
`tableWithSpans'`, `toTableComponentsWithSpans` and
`toTableComponentsWithSpans'` take a list of lists of
(Blocks, RowSpan, ColSpan) to parse a Table with different RowSpan and
ColSpan values accordingly.
New helper functions `singleRowSpans` and `singleColumnSpans` help set
all RowSpans or ColSpans to be 1 in case the table format only allows
setting one or the other.