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, 1 deletions

diff --git a/rows.go b/rows.go
index 7de18d3..ff268cf 100644
--- a/rows.go
+++ b/rows.go
@@ -446,7 +446,7 @@ func (f *File) InsertRow(sheet string, row int) error {
 	return f.adjustHelper(sheet, rows, row, 1)
 }
 
-// DuplicateRow inserts a copy of specified row (by it Excel row number) below
+// DuplicateRow inserts a copy of specified row (by its Excel row number) below
 //
 //    err := xlsx.DuplicateRow("Sheet1", 2)
 //