turbo-tasks: adopt triomphe::Arc for CachedTaskType
Migrate Arc<CachedTaskType> to triomphe::Arc via CachedTaskTypeArc newtype.
Saves one usize per allocation (no weak count) and avoids the weak-count
CAS in drop_slow compared to std::sync::Arc. We never need
Weak<CachedTaskType>, so the trade-off is favorable.
