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)
diff --git a/src/proto.h b/src/proto.h
index 0c5a71f..c301857 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -75,7 +75,10 @@
 # include "digraph.pro"
 # include "edit.pro"
 # include "eval.pro"
+# include "evalbuffer.pro"
 # include "evalfunc.pro"
+# include "evalvars.pro"
+# include "evalwindow.pro"
 # include "ex_cmds.pro"
 # include "ex_cmds2.pro"
 # include "ex_docmd.pro"
@@ -111,7 +114,6 @@
 # ifdef FEAT_VIMINFO
 # include "viminfo.pro"
 # endif
-# include "evalvars.pro"
 
 /* These prototypes cannot be produced automatically. */
 int smsg(const char *, ...)