Make backstep calculation non-recursive.

Refs https://github.com/boostorg/regex/pull/236.

test/lookbehind_recursion_stress_test.cpp

@@ -0,0 +1,28 @@
+/*
+ *
+ * Copyright (c) 2024
+ * John Maddock
+ *
+ * Use, modification and distribution are subject to the
+ * Boost Software License, Version 1.0. (See accompanying file
+ * LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ *
+ */
+
+#include <boost/regex.hpp>
+
+int main()
+{
+   std::string s("(?<=(");
+   s.append(1000, '|');
+   s += "))";
+   boost::regex rx(s);
+
+   s = "(?<=(a";
+   for (unsigned i = 0; i < 1000; ++i)
+   {
+      s += "|a";
+   }
+   s += "))";
+   boost::regex rx2(s);
+}