Realtime-safe channels (lock-free queues) for C++20.

These channels are queues backed by a ring buffer. They use only atomic operations and do not use mutex, semaphores, etc. and are suitable for use in real-time applications. These channels support batched operations, as well as expose the underlying buffer directly, which is rather useful when computing and sending large chunks of data, such as audio buffers.

rtchan is a work-in-progress and is not yet suitable for use of any kind.