C++26: Removed and Deprecated Features Roundup
C++26 is removing or deprecating several features. These include the complete removal of the `std::allocator` typedef deprecated in C++20, and the no-argument overload of `std::basic_string::reserve()`; removal of deprecated Unicode conversion utilities and `std::strtok`; removal of aged `strstreams` and `std::shared_ptr` atomic access APIs; and removal of `std::wstring_convert`. Additionally, `std::is_trivial` is deprecated, with suggestions to use the more precise `is_trivially_XXX` alternatives; and `std::memory_order::consume` is deprecated due to unsatisfactory specification and implementation difficulties. These removals and deprecations aim to improve language safety and efficiency, and clean up outdated functionality.