`Scheduler` should be thread-safe

[jacnel]

The current implementation of Scheduler does not allow tasks to be scheduled after the scheduler is started. Ideally, it could be run in a different thread and will continuously run new coroutines as they are added.

Design idea:

• Have a queue to represent coroutines pending scheduling
• Whenever a coroutine gives up execution, check if any additional coroutines need to be scheduled and add them to the active list if there are