meta/folly ? reverse-engineered prompt

Build me a C++20 foundational utility library like Folly, with a clean public header layout, a top level folly namespace, and a CMake build that can compile the library and optional tests on Linux and macOS.

I want a practical set of components that fill common gaps in the standard library, things like fast containers, concurrency helpers, futures and promises, a simple async event loop, byte buffers, string and number conversion helpers, scope guards, and a small benchmark utility. Keep the code focused on performance and readability, and make the pieces work together without forcing a big complicated framework.

Please include a few example tests and enough documentation comments so it is clear how to use the main pieces. If something depends on optional system libraries, make it build cleanly when they are missing. Use current CMake best practices, and look up any recent docs online if you need to.