patch 8.1.2004: 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_substitute.vim b/src/testdir/test_substitute.vim
index a263fb6..8a8e7ab 100644
--- a/src/testdir/test_substitute.vim
+++ b/src/testdir/test_substitute.vim
@@ -337,7 +337,7 @@
 		\ substitute('A123456789',
 		\ 'A\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)',
 		\ '\=string([submatch(0, 1), submatch(9, 1), ' .
-		\ 'submatch(8, 1), submatch(7, 1), submatch(6, 1), ' .
+		\ 'submatch(8, 1), 7->submatch(1), submatch(6, 1), ' .
 		\ 'submatch(5, 1), submatch(4, 1), submatch(3, 1), ' .
 		\ 'submatch(2, 1), submatch(1, 1)])',
 		\ ''))