Incorporate TooSlowError into CancelScope

[jakkdl]

After #3010 we will have the following functionality:

my_move_on_at = move_on_at(5)
assert my_move_on_at.deadline == 5
my_move_on_after = move_on_after(5)
assert my_move_on_after.deadline == 5

# but these return Generator[CancelScope], so you can't inspect/modify the objects
my_fail_at = fail_at(5)
assert my_fail_at.deadline == 5 # AttributeError
my_fail_after = fail_after(5)
assert my_fail_after.deadline == 5 # AttributeError

Since separating initialization & entry has been weird previously the current demand might be low for this functionality, but it might become more common.
The two ways I see of fixing this are:

1. Add a wrapper class that defines __exit__ to raise TooSlowError if cancelled_caught, but otherwise passes __getattr__ and __setattr__ onto CancelScope
2. Add a fail=True parameter to CancelScope that makes it raise TooSlowError upon being cancelled.

Split out from #3010 (comment)