patch 9.1.1137: ins_str() is inefficient by calling STRLEN()

Problem:  ins_str() is inefficient by calling STRLLEN()
Solution: refactor ins_str() to take a length argument
          and let all callers provide the correct length
          when calling ins_str() (John Marriott)

closes: #16711

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/textformat.c b/src/textformat.c
index 77e3eef..018565f 100644
--- a/src/textformat.c
+++ b/src/textformat.c
@@ -434,7 +434,7 @@
 			// add the additional whitespace needed after the
 			// comment leader for the numbered list.
 			for (i = 0; i < padding; i++)
-			    ins_str((char_u *)" ");
+			    ins_str((char_u *)" ", 1);
 		    }
 		    else
 		    {