Lower the row collection scrubbing in Fast Forward mode

[jandrew]

This is valuable when a person wants to read the middle/end of the sheet only.

The change would be applied to the ~::WorksheetToRow module

This could be accomplished with two things. First only reading the row number when cycling through the sheet and storing the last and current row informally until the desired row position is reached and then scrub and store both prior and current rows as Row objects. Second, build a row to position lookup JIT so that jumping around in the sheet would be marginally less painful after the first parse that included the jump point.

[jandrew]

After contemplating this implementation maybe not storing the last row at all just the last row number is best. I'm not sure how often people jump back and forth between two rows vs wanting to fast forward to the middle of a sheet. The second case seems more likely and in my personal usage the more important.