`tmc::shared_mutex` - async version of `std::shared_mutex` (aka RWLock)

[tzcnt]

Enhance the suite of control structures by adding an async analogue to std::shared_mutex, which is a form of reader-writer lock.

[dixlorenz]

If you are looking for ideas for more control structures: mutex is comparable to ex_braid, which compares to Apple's GCD serial_queue. a shared_mutex would compare to a concurrent_queue (which I find a really cool concept somehow). Not sure if it could be an executor because queuing anything on there needs information if it is a barrier function or not; maybe there could a default when used as an executor?
OTOH, maybe somebody should build some higher level concurrency structures outside of this repo, mimicking queues/dispatchgroups of GCD, based on and interoperable with tmc, like tmc-asio? sounds like a fun project to me; might play around with it ;-)