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_expand_func.vim b/src/testdir/test_expand_func.vim
index f6e5293..f9c5b5f 100644
--- a/src/testdir/test_expand_func.vim
+++ b/src/testdir/test_expand_func.vim
@@ -68,7 +68,7 @@
 func Test_expand()
   new
   call assert_equal("",  expand('%:S'))
-  call assert_equal('3', expand('<slnum>'))
+  call assert_equal('3', '<slnum>'->expand())
   call assert_equal(['4'], expand('<slnum>', v:false, v:true))
   " Don't add any line above this, otherwise <slnum> will change.
   quit