patch 9.1.0256: Finding autocmd events is inefficient

Problem:  Finding autocmd events is inefficient
Solution: Use binary search to find events, cache last found events,
          avoid use of strlen(), add SessionWritePost autocmd,
          fix test_codestyle and avoid endless loop
          (John Marriott)

closes: #14287

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    256,
 /**/
     255,
 /**/