Make ast::Node::accept() const

Visitors currently can't mutate the AST anyway. Fixes #56.

Author: swolchok

Diff for: AstNode.h

@@ -30,7 +30,7 @@ class Node {
   const yy::location &getLocation() const
   { return location_; }
 
-  virtual void accept(visitor::AstVisitor *visitor) = 0;
+  virtual void accept(visitor::AstVisitor *visitor) const = 0;
 };
 
 }

Diff for: ast/cxx.py

@@ -77,7 +77,7 @@ def end_type(self, name):
     self._print_noncopyable()
     print >> self._deferredOutput
     self._print_getters()
-    print >> self._deferredOutput, '  void accept(visitor::AstVisitor *visitor) override;'
+    print >> self._deferredOutput, '  void accept(visitor::AstVisitor *visitor) const override;'
     print >> self._deferredOutput, '};'
     print >> self._deferredOutput
     print >> self._deferredOutput

Diff for: ast/cxx_impl.py

@@ -28,7 +28,7 @@ def end_file(self):
     print '}'
 
   def start_type(self, name):
-    print '''void %s::accept(visitor::AstVisitor *visitor) {
+    print '''void %s::accept(visitor::AstVisitor *visitor) const {
   if (visitor->visit%s(*this)) {
 ''' % (name, name)