Better expose Nursery and TaskStatus for type annotations

[goodboy]

As per some gitter banter we should more publicly expose the Nursery, CancelScope and Taskstatus types for use as type annotations.

Currently client code is stuck importing most of these from internal private modules which leads the following beauty:

async def _async_main(                                                                                                                                                                              
   self,                                                                                                                                                                                           
   accept_addr: Tuple[str, int],                                                                                                                                                                   
   arbiter_addr: Optional[Tuple[str, int]] = None,                                                                                                                                                 
   parent_addr: Optional[Tuple[str, int]] = None,                                                                                                                                                  
   task_status: trio._core._run._TaskStatus = trio.TASK_STATUS_IGNORED,                                                                                                                            
) -> None:
     ...

Worth noting:

• CancelScope may already be exposed soon via idea: unbound cancel scopes #607
• @njsmith and @Fuyukai already seems interested in adding them internally as per Provide PEP 484-style type hints #543

[goodboy]

Oh, and let's not forget about the very important Task!

[oremanj]

Task is already public as trio.hazmat.Task -- how were you envisioning it getting more public?

For now I recommend using the Nursery and TaskStatus exposures in trio-typing. When we bring the type hints into Trio proper (#543), I imagine a resolution to this issue will go along with it. I'm going to close this for now, but leave a pointer at #543.