struct ThreadToken

ThreadToken and ScopedThreadToken are small (empty) objects which are intended to be used with

the clang static thread analysis framework in order to express that some data may only be

accessed from a single thread. By obtaining the capability represented by a threads token in an

async operation submitted to that threads dispatcher, users may assert at compile time that they

are only touching members from a single thread.

This requires that the `async_dispatcher_t` backing any async waits is single threaded since this

class does not do any actual locking.