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/rationale.adoc

@@ -121,7 +121,8 @@ for Visual Studio on an x64-mode Intel CPU with SSE2 and for GCC on an IBM s390x
 == Concurrent Containers
 
 The same data structure used by Boost.Unordered open-addressing containers has been chosen
-also as the foundation of `boost::concurrent_flat_set` and `boost::concurrent_flat_map`:
+also as the foundation of `boost::concurrent_flat_set`/`boost::concurrent_node_set` and
+`boost::concurrent_flat_map`/`boost::concurrent_node_map`:
 
 * Open-addressing is faster than closed-addressing alternatives, both in non-concurrent and
 concurrent scenarios.
@@ -130,7 +131,7 @@ with minimal locking. In particular, the metadata array can be used for implemen
 lookup that are lock-free up to the last step of actual element comparison.
 * Layout compatibility with Boost.Unordered flat containers allows for 
 xref:#concurrent_interoperability_with_non_concurrent_containers[fast transfer]
-of all elements between `boost::concurrent_flat_map` and `boost::unordered_flat_map`,
+of all elements between a concurrent container and its non-concurrent counterpart,
 and vice versa.
 
 === Hash Function and Platform Interoperability