CancelScope should ensure that the current task is the same in __enter__() and __exit__()

The following snippet runs without errors and it shouldn't:

import trio


async def enter_scope(scope):
    scope.__enter__()


async def exit_scope(scope):
    scope.__exit__(None, None, None)


async def main():
    scope = trio.CancelScope()
    async with trio.open_nursery() as nursery:
        nursery.start_soon(enter_scope, scope)
    async with trio.open_nursery() as nursery:
        nursery.start_soon(exit_scope, scope)

trio.run(main)

It should check that the current task in __enter__() and __exit__() are the same.

Start by reproducing the provided snippet and then inspect CancelScope.enter() and exit(). The change is complete when entering in one task and exiting in another is rejected, while normal same-task use continues to work.