Comments on the rendered view of Markdown attachments store row numbers.

I'm going to be changing the row layout to be block-based, and comments

made on the old layout will need to remap to the matching rows of the

new one.
This change adds two new helpers for this:


get_row_text_ranges() returns where each row's text sits in the

  whole document. Whitespace isn't counted, so different layouts of the

  same Markdown line up. Images and horizontal rules count as one

  character, and each line of code gets one extra character so empty

  lines still have a position.


map_text_ranges() maps each row of one layout to the rows of another

  that its text overlaps. This handles rows that were merged (lines of a

  paragraph) and rows that were split (rows of a table). Rows without

  text map to the row before them.


Nothing uses these yet.

Ran unit tests.