patch 8.1.2001: some source files are too big

Problem:    Some source files are too big.
Solution:   Move buffer and window related functions to evalbuffer.c and
            evalwindow.c. (Yegappan Lakshmanan, closes #4898)

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2001,
 /**/
     1999,
 /**/