* 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)
