Tighten the assertions around yielded tasks.
Tighten the assertions around yielded tasks.

This also fixes a potential issue when Task.join() is used from outside of a task (it could then remain in the m_yielders queue after it had called yield()).

Geod24/vibe.d,chalucha/vibe.d,rejectedsoftware/vibe.d,Mihail-K/vibe.d,DBankov/vibe.d,etcimon/vibe.d,redstar/vibe.d,machindertechnologies/vibe.d,AndreyMZ/vibe.d,etcimon/vibe.d,gedaiu/vibe.d,schuetzm/vibe.d