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_matchadd_conceal_utf8.vim b/src/testdir/test_matchadd_conceal_utf8.vim
index 98ed2cb..2cdb062 100644
--- a/src/testdir/test_matchadd_conceal_utf8.vim
+++ b/src/testdir/test_matchadd_conceal_utf8.vim
@@ -10,7 +10,7 @@
 func s:screenline(lnum) abort
   let line = []
   for c in range(1, winwidth(0))
-    call add(line, nr2char(screenchar(a:lnum, c)))
+    call add(line, nr2char(a:lnum->screenchar(c)))
   endfor
   return s:trim(join(line, ''))
 endfunc