patch 8.1.1809: more functions can be used as a method

Problem:    More functions can be used as a method.
Solution:   Add has_key(), split(), str2list(), etc.
diff --git a/src/testdir/test_system.vim b/src/testdir/test_system.vim
index 25061ea..f09f509 100644
--- a/src/testdir/test_system.vim
+++ b/src/testdir/test_system.vim
@@ -4,13 +4,13 @@
   if !executable('echo') || !executable('cat') || !executable('wc')
     return
   endif
-  let out = system('echo 123')
+  let out = 'echo 123'->system()
   " On Windows we may get a trailing space.
   if out != "123 \n"
     call assert_equal("123\n", out)
   endif
 
-  let out = systemlist('echo 123')
+  let out = 'echo 123'->systemlist()
   " On Windows we may get a trailing space and CR.
   if out != ["123 \r"]
     call assert_equal(['123'], out)