patch 8.1.1993: 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_utf8.vim b/src/testdir/test_utf8.vim
index 9470855..fa0568e 100644
--- a/src/testdir/test_utf8.vim
+++ b/src/testdir/test_utf8.vim
@@ -112,7 +112,7 @@
   call setline(1, ["ABC\u0308"])
   redraw
   call assert_equal([0x0041], screenchars(1, 1))
-  call assert_equal([0x0042], screenchars(1, 2))
+  call assert_equal([0x0042], 1->screenchars(2))
   call assert_equal([0x0043, 0x0308], screenchars(1, 3))
   call assert_equal("A", screenstring(1, 1))
   call assert_equal("B", screenstring(1, 2))