Update call to mrb_integer_to_str in mrbsys cext

With the `Fixnum` deprecation in mruby 3.1.0, `mrb_fixnum_to_str` was
renamed to `mrb_integer_to_str`. `mrb_fixnum_to_str` was retained as a
macro alias for source compatibility.

Update to the current stable function name.

artichoke-backend/cext/mrbsys/src/ext.c

@@ -378,7 +378,7 @@ mrb_obj_as_string(mrb_state *mrb, mrb_value obj)
     case MRB_TT_SYMBOL:
       return mrb_sym_str(mrb, mrb_symbol(obj));
     case MRB_TT_INTEGER:
-      return mrb_fixnum_to_str(mrb, obj, 10);
+      return mrb_integer_to_str(mrb, obj, 10);
     case MRB_TT_SCLASS:
     case MRB_TT_CLASS:
     case MRB_TT_MODULE: