string formatter follow srfi-28's spec

src/codegen.c

@@ -304,7 +304,7 @@ analyze_node(analyze_state *state, pic_value obj, bool tailpos)
     pic_value proc;
 
     if (! pic_list_p(pic, obj)) {
-      pic_errorf(pic, "invalid expression given: ~S", obj);
+      pic_errorf(pic, "invalid expression given: ~s", obj);
     }
 
     proc = pic_list_ref(pic, obj, 0);

src/string.c

@@ -188,7 +188,7 @@ pic_vfformat(pic_state *pic, xFILE *file, const char *fmt, va_list ap)
       case '%':
         xfputc('\n', file);
         break;
-      case 'S':
+      case 's':
         irrs = pic_cons(pic, pic_fdebug(pic, va_arg(ap, pic_value), file), irrs);
         break;
       }

src/vm.c

@@ -558,7 +558,7 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value argv)
     L_CALL:
       x = pic->sp[-c.u.i];
       if (! pic_proc_p(x)) {
-	pic_errorf(pic, "invalid application: ~S", x);
+	pic_errorf(pic, "invalid application: ~s", x);
       }
       proc = pic_proc_ptr(x);