patch 8.1.1989: the evalfunc.c file is still too big

Problem:    The evalfunc.c file is still too big.
Solution:   Move f_pathshorten() to filepath.c.  Move f_cscope_connection() to
            if_cscope.c.  Move diff_ functions to diff.c.  Move timer_
            functions to ex_cmds2.c.  move callback functions to evalvars.c.
diff --git a/src/proto/evalvars.pro b/src/proto/evalvars.pro
index 998246d..691eec3 100644
--- a/src/proto/evalvars.pro
+++ b/src/proto/evalvars.pro
@@ -77,4 +77,8 @@
 void f_settabwinvar(typval_T *argvars, typval_T *rettv);
 void f_setwinvar(typval_T *argvars, typval_T *rettv);
 void f_setbufvar(typval_T *argvars, typval_T *rettv);
+callback_T get_callback(typval_T *arg);
+void put_callback(callback_T *cb, typval_T *tv);
+void set_callback(callback_T *dest, callback_T *src);
+void free_callback(callback_T *callback);
 /* vim: set ft=c : */