Remove limit on the number of backrefs possible.

Changes named sub-expressions to use different hashing scheme: high order bit is now always set to clashes between hashes and indexes don't happen until 2^30 or 2^62 sub-expressions in 32 and 64 bit code respectively.
Changes bitmask of seen sub-expressions to use dynamic storage for sub-expression indexes above 64.
Adds tests for the above.
Fixes https://github.com/boostorg/regex/issues/75.

include/boost/regex/v4/indexed_bit_flag.hpp

@@ -0,0 +1,54 @@
+/*
+ *
+ * Copyright (c) 2020
+ * 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)
+ *
+ */
+
+ /*
+  *   LOCATION:    see http://www.boost.org for most recent version.
+  *   FILE         basic_regex_parser.cpp
+  *   VERSION      see <boost/version.hpp>
+  *   DESCRIPTION: Declares template class basic_regex_parser.
+  */
+
+#include <boost/regex/config.hpp>
+#include <set>
+
+#ifndef BOOST_REGEX_V4_INDEXED_BIT_FLAG_HPP
+#define BOOST_REGEX_V4_INDEXED_BIT_FLAG_HPP
+
+namespace boost{
+namespace BOOST_REGEX_DETAIL_NS{
+
+class indexed_bit_flag
+{
+   boost::uint64_t low_mask;
+   std::set<std::size_t> mask_set;
+public:
+   indexed_bit_flag() : low_mask(0) {}
+   void set(std::size_t i)
+   {
+      if (i < std::numeric_limits<boost::uint64_t>::digits - 1)
+         low_mask |= static_cast<boost::uint64_t>(1u) << i;
+      else
+         mask_set.insert(i);
+   }
+   bool test(std::size_t i)
+   {
+      if (i < std::numeric_limits<boost::uint64_t>::digits - 1)
+         return low_mask & static_cast<boost::uint64_t>(1u) << i ? true : false;
+      else
+         return mask_set.find(i) != mask_set.end();
+   }
+};
+
+} // namespace BOOST_REGEX_DETAIL_NS
+} // namespace boost
+
+
+#endif