Define concurrency model

[DavidLeoni]

Seems async / await are not so bad after all, I found out I can avoid async keyword poisoning - got some inspiration from Textual api

Will provide double use api: by default execution will be non-blocking, users will be optionally able to use await:

Default, non-blocking:

ada = Sprite()
ada.say("hello",3)
print("already done!")

Optional blocking behaviour:

ada = Sprite()
await ada.say("hello",3)
print("... after some time!")

Syncing between methods calls can still be done with usual asyncio functions, like gather:

ada = Sprite()
bob = Sprite()
await asyncio.gather(ada.slide(-100,0, 5), 
                                  bob.slide(-100,0, 3))
print("already done!")

Things still to consider:

• Screen.register_shape and Screen.bgpic: in some way we should wait all resources are loaded
• tracing in goto commands
• normal methods, students may inadvertently treat them as awaitable