vim-patch:8.1.1984: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
3f4f3d8e7e

test_prompt_buffer.vim already had all the changes, except
Test_prompt_garbage_collect().

src/nvim/testdir/test_python2.vim

@@ -59,7 +59,7 @@ func Test_vim_function()
 
   try
     py f = vim.Function('\x80\xfdR' + vim.eval('s:foo()'))
-    call assert_equal(name, pyeval('f.name'))
+    call assert_equal(name, 'f.name'->pyeval())
   catch
     call assert_false(v:exception)
   endtry