Fix C++17 deprecation warning from std::allocator::is_always_equal

[edsavage]

When compiling with support for C++17 the following deprecation warning is encountered

C:\usr\local\include\boost-1_77\boost/unordered/detail/implementation.hpp(1452,7): warning C4996: 'std::allocator<std::pair<const std::basic_string<char,std::char_traits<char>,std::allocator<char>>,std::basic_string<char,std::char_traits<char>,std::allocator<char>>>>::is_always_equal': warning STL4010: Various members of std::allocator are deprecated in C++17. Use std::allocator_traits instead of accessing these members directly. You can define _SILENCE_CXX17_OLD_ALLOCATOR_MEMBERS_DEPRECATION_WARNING or _SILENCE_ALL_CXX17_DEPRECATION_WARNINGS to acknowledge that you have received this warning.

Following the lead suggested here
https://groups.google.com/g/boost-developers-archive/c/3MOWuQgEEy4/m/boIwsJsdBAAJ
and using boost::allocator_is_always_equal from
<boost/core/allocator_access.hpp> does fix the source of the deprecation warning.

[glenfe]

All of the Unordered allocator traits machinery should be removed and replaced with <boost/core/allocator_access.hpp> which simplifies Unordered's implementation drastically. @LeonineKing1199 is working on this.

[glenfe]

@LeonineKing1199 can you confirm that this is now fixed in develop?

[cmazakas]

@glenfe @edsavage

I can happily confirm that the deprecation warning no longer appears!

For example, consider the logs from AppVeyor here which is a slightly older build.

Compare it to the logs in the latest AppVeyor run here.

No more warnings!

If you wish to confirm yourself, I'd recommend downloading the logs themselves as the AppVeyor site seems to struggle with rendering that much output whereas downloading the logs and opening them with a minimal text editor is quite fast.

[edsavage]

Closing this PR as this the issue it was trying to fix has been addressed in a better way.