Resolve #382, rewrite prepareSheetXML to scale linearly (#383)

* Rewrite prepareSheetXML to scale linearly

We don't need to backfill columns into every row for most purposes
Provided makeContiguousColumns for setting styles where we do
need it for a specific region.

Added a benchmark to monitor progress. For 50,000 rows this went
from about 11 seconds to 1 second. The improvements are more
dramatic as the row/column count increases.

* Assigning that row value was redundant

1 files changed, 1 insertions, 0 deletions

diff --git a/styles.go b/styles.go
index a515756..81d03be 100644
--- a/styles.go
+++ b/styles.go
@@ -2373,6 +2373,7 @@ func (f *File) SetCellStyle(sheet, hcell, vcell string, styleID int) error {
 		return err
 	}
 	prepareSheetXML(xlsx, vcol, vrow)
+	makeContiguousColumns(xlsx, hrow, vrow, vcol)
 
 	for r := hrowIdx; r <= vrowIdx; r++ {
 		for k := hcolIdx; k <= vcolIdx; k++ {