Issue 4550: Avoid "using namespace std;" in included files (Take 2)

lily/mensural-ligature.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 2002--2020 Juergen Reuter <reuter@ipd.uka.de>,
   Pal Benko <benkop@freestart.hu>
 
   LilyPond is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.
@@ skipping 12 matching lines @@
 #include "directional-element-interface.hh"
 #include "font-interface.hh"
 #include "international.hh"
 #include "item.hh"
 #include "lookup.hh"
 #include "note-head.hh"
 #include "output-def.hh"
 #include "staff-symbol-referencer.hh"
 #include "warn.hh"
 
+using std::string;
+
 /*
   draws one half a flexa, i.e. a portion corresponding to a single note.
   this way coloration of the two notes building up the flexa can be
   handled independently.
 
  * TODO: divide this function into mensural and neo-mensural style.
  *
  * TODO: move this function to class Lookup?
  */
 Stencil
@@ skipping 245 matching lines @@
 
                /* properties */
                "delta-position "
                "ligature-flexa "
                "head-width "
                "add-join "
                "flexa-interval "
                "primitive "
                "thickness "
               );