Feature/concurrent node containers (#271)

* added concurrent node containers

* removed spurious typename

* added missing includes

* avoided unused param warning

* worked around Clang bug

* s/{}/() to work around GCC4.8 problems with aggregate initialization

* used /bigobj for cfoa/visit_tests.cpp

* suppressed localized maybe-uninitialized warnings

* fixed comments

* added /bigobj to cfoa/insert_tests.cpp

* instrumented double exact comparison to spot a spurious error

* fixed pedantic error

* refactored byte_span machinery

* compromised on sub-epsilon equality for doubles that should be identical

* documented boost::concurrent_node_(map|set)

* added concurrent_node_set

* added missing AlternativeType

* tested empty node insertion

* tested node_handle allocator management

* added nonassignable_allocator and node_handle_allocator_swap_tests

* fixed warning disabling

* silenced spurious GCC warning

* broadened scope of previous pragma

* broadened even more

* worked around spurious constexpr-related msvc-14.0 bug
https://godbolt.org/z/v78545Ebf

* added workaround back

* replaced previous workaround with built-in one

* added workaround back on top of built-in solution (which doesn't work 100% of the time)

test/cfoa/exception_erase_tests.cpp

@@ -1,5 +1,5 @@
 // Copyright (C) 2023 Christian Mazakas
-// Copyright (C) 2023 Joaquin M Lopez Munoz
+// Copyright (C) 2023-2024 Joaquin M Lopez Munoz
 // Distributed under 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)
 
@@ -7,6 +7,8 @@
 
 #include <boost/unordered/concurrent_flat_map.hpp>
 #include <boost/unordered/concurrent_flat_set.hpp>
+#include <boost/unordered/concurrent_node_map.hpp>
+#include <boost/unordered/concurrent_node_set.hpp>
 
 #include <boost/core/ignore_unused.hpp>
 
@@ -283,8 +285,12 @@ namespace {
 
   boost::unordered::concurrent_flat_map<raii, raii, stateful_hash,
     stateful_key_equal, stateful_allocator<std::pair<raii const, raii> > >* map;
+  boost::unordered::concurrent_node_map<raii, raii, stateful_hash,
+    stateful_key_equal, stateful_allocator<std::pair<raii const, raii> > >* node_map;
   boost::unordered::concurrent_flat_set<raii, stateful_hash,
     stateful_key_equal, stateful_allocator<raii> >* set;
+  boost::unordered::concurrent_node_set<raii, stateful_hash,
+    stateful_key_equal, stateful_allocator<raii> >* node_set;
 
 } // namespace
 
@@ -295,7 +301,7 @@ using test::sequential;
 // clang-format off
 UNORDERED_TEST(
   erase,
-  ((map)(set))
+  ((map)(node_map)(set)(node_set))
   ((exception_value_type_generator_factory)
    (exception_init_type_generator_factory))
   ((lvalue_eraser)(lvalue_eraser_if)(erase_if)(free_fn_erase_if))