More robust CancelScope misnesting prevention

[A5rocks]

@agronholm brought this case up on Gitter:

import trio

async def main():
    async def shim(fn, *args):
        fn(*args)

    scope = trio.CancelScope()
    async with trio.open_nursery() as nursery:
        nursery.start_soon(shim, scope.__enter__)

    async with trio.open_nursery() as nursery:
        nursery.start_soon(shim, scope.__exit__, None, None, None)

trio.run(main)

I think we should probably error here. I took a look at the code and I think the issue is that our CancelScope misnesting prevention only works within a single task -- that is, we don't check that there's no CancelScope currently entered at the end of any task. Unfortunately, IIRC that's hard in that situation to raise an exception with a nice stack trace. As a stopgap, we could raise when exiting the nursery if anything gets marked as misnested. (... which would raise later but should catch all of these misnestings?)

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I'm making mostly unsubstantiated conjectures here, based on a few minutes of playing around with the code. There's very likely something I missed!