Design: task supervision and results

[njsmith]

Ugh, this is a big open area.

I'm somewhat hesitant to build a specific supervision framework into trio right away, because it feels like an area that needs some exploration. OTOH I guess trio is unstable, so whatever? But what I'm most concerned about is getting the underlying APIs right; I'm pretty sure we want some lower level API to exist beyond the actual supervisor system.

I really don't like the current .join() design though, because it's way too easy to write task.join() and now you've just swallowed an exception.

[njsmith]

Solved by the addition of the nursery system.