Add a `.timeout` attribute to `CancelScope`?

[njsmith]

Now that CancelScopes are becoming more exposed to users with #607 / #835, it occurred to me that it might be nice to have a weird magical .timeout property on them. The definition would be:

@property
def timeout(self):
    return max(0.0, self._deadline - trio.current_time())

@timeout.setter
def timeout(self, new_timeout):
    self.deadline = trio.current_time() + new_timeout

[njsmith]

...Of course as soon as I posted that I realized there's a probably-fatal flaw. This code looks all nice and sensible:

cancel_scope.timeout += 1

But with the definition above, it would call current_time() twice, so it would actually cause the deadline to increase by slightly more than 1 second, which seems like a super obscure gotcha.

[smurfix]

Well, we could always create .timeout as a magical int-derived object that actually supports iadd by updating its parent … though that would have interesting overhead (and require a weakref). Let's just add a .timeout_after method (or whatever the name shall be) instead.

[njsmith]

I'm not convinced there's anything better to do here than what we're already doing. We can always re-open later when we have more experience with the new CancelScope APIs.