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_bufline.vim b/src/testdir/test_bufline.vim
index 183a186..50f7275 100644
--- a/src/testdir/test_bufline.vim
+++ b/src/testdir/test_bufline.vim
@@ -22,7 +22,7 @@
   call assert_equal(1, setbufline(b, 5, ['x']))
   call assert_equal(1, setbufline(bufnr('$') + 1, 1, ['x']))
   call assert_equal(0, setbufline(b, 4, ['d', 'e']))
-  call assert_equal(['c'], getbufline(b, 3))
+  call assert_equal(['c'], b->getbufline(3))
   call assert_equal(['d'], getbufline(b, 4))
   call assert_equal(['e'], getbufline(b, 5))
   call assert_equal([], getbufline(b, 6))