patch 8.1.1987: 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_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index 82f9cac..b518f5a 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
@@ -75,7 +75,7 @@
   call setline(1, ['one', 'two', 'three'])
   setlocal ff=dos
   write XReadfile
-  let lines = readfile('XReadfile')
+  let lines = 'XReadfile'->readfile()
   call assert_equal(['one', 'two', 'three'], lines)
   let lines = readfile('XReadfile', '', 2)
   call assert_equal(['one', 'two'], lines)