patch 8.1.1921: 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_fnameescape.vim b/src/testdir/test_fnameescape.vim
index 5382b89..b0fdb3a 100644
--- a/src/testdir/test_fnameescape.vim
+++ b/src/testdir/test_fnameescape.vim
@@ -13,7 +13,7 @@
   let fname = 'Xemark!'
   let status = v:false
   try
-    exe "w! " . fnameescape(fname)
+    exe "w! " . fname->fnameescape()
     let status = v:true
   endtry
   call assert_true(status, "ExclamationMark")