Scheduler should drop unrunnable tasks into the work queue instead of blocking on them

[larsbergstrom]

See the example code shown in:
https://gist.github.com/ILyoan/6945608

The problem seems to be that when we use start_on_main_thread, any tasks subsequently spawned may be sent to the friend single threaded scheduler via a message, but they will not be made available for pickup by the work-stealing scheduler. This design prevents parallel speedups for the gist as written above. If it's switched to instead start the program from the normal #[main], spawn works more closely to expectations.

[Aatch]

Looks like this was closed by #9857