The coroutine callee type, used by the implementation.
typedef implementation_defined callee_type;
When using Boost.Coroutine v1, this type is:
typename coroutine<void()>
When using Boost.Coroutine v2 (unidirectional coroutines), this type is:
push_coroutine<void>
Header: asio/spawn.hpp
Convenience header: None