patch 8.1.1925: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
diff --git a/src/testdir/test_file_perm.vim b/src/testdir/test_file_perm.vim
index 0dba6a7..3950168 100644
--- a/src/testdir/test_file_perm.vim
+++ b/src/testdir/test_file_perm.vim
@@ -2,10 +2,10 @@
 
 func Test_file_perm()
   call assert_equal('', getfperm('Xtest'))
-  call assert_equal(0, setfperm('Xtest', 'r--------'))
+  call assert_equal(0, 'Xtest'->setfperm('r--------'))
 
   call writefile(['one'], 'Xtest')
-  call assert_true(len(getfperm('Xtest')) == 9)
+  call assert_true(len('Xtest'->getfperm()) == 9)
 
   call assert_equal(1, setfperm('Xtest', 'rwx------'))
   if has('win32')