Use colon instead of as to cast

boot/lexer.ll

@@ -134,9 +134,6 @@ return                  {
 cast                    {
                             return yy::parser::make_CAST(this->location);
                         }
-as                      {
-                            return yy::parser::make_AS(this->location);
-                        }
 sizeof                  {
                             return yy::parser::make_SIZEOF(this->location);
                         }