Worker implementation

[Sygmei]

Workers are independant Lua states in a thread.

They can be used to perform CPU intensive tasks or IO blocking tasks without coroutine support.

Since it lives in an independant thread, it will not block the main loop.
Workers can live in their thread alone or share it with other Workers (1 state per thread or multiple states per thread).

Workers could be represented as either a Scene/GameObject or a Component (such as Sprite / Collider) or a simple userdata that lives inside an existing Lua state. We need to decide what would be the most logical choice.

Events can be used to communicate with the Worker and we could have default events such as "Completed"