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)

doc/unordered/changes.adoc

@@ -6,8 +6,9 @@
 :github-pr-url: https://github.com/boostorg/unordered/pull
 :cpp: C++
 
-== Release 1.87.0
+== Release 1.87.0 - Major update
 
+* Added concurrent, node-based containers `boost::concurrent_node_map` and `boost::concurrent_node_set`.
 * Made visitation exclusive-locked within certain
 `boost::concurrent_flat_set` operations to allow for safe mutable modification of elements
 ({github-pr-url}/265[PR#265^]).