The casual reader, on encountering these in the Fiber examples directory,
might otherwise assume they're intended to illustrate something about the
Fiber library.
In essence, yield_t et al. become very like use_future_t et al. The only real
difference is the async_result return type and get() method. Factor out common
functionality into promise_completion_token and promise_handler.
Remove the boost::fibers::asio::spawn() function and its basic_yield_context
infrastructure. Fix up all existing references in example source code.
