bitset: optimize the code generated by shifts in common cases

* spot/misc/bitset.hh: Here.
* spot/misc/bitset.cc: New file.
* spot/misc/Makefile.am: Add it.
* spot/twa/acc.hh: Conditionally remove the exception checks around
shift operators.
* spot/misc/common.hh (SPOT_ASSUME): New macro.

spot/misc/bitset.cc

@@ -0,0 +1,32 @@
+// -*- coding: utf-8 -*-
+// Copyright (C) 2018 Laboratoire de Recherche et Développement
+// de l'Epita (LRDE).
+//
+// This file is part of Spot, a model checking library.
+//
+// Spot 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.
+//
+// Spot is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+// or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+// License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#include "config.h"
+#include <spot/misc/bitset.hh>
+
+namespace spot
+{
+  namespace internal
+  {
+    void report_bit_shift_too_big()
+    {
+      throw std::runtime_error("bit shift by more bits than supported");
+    }
+  }
+}