Rework the API for I/O safety

[notgull]

This reworks the polling API with I/O safety in mind. See #38 and smol-rs/smol#244 for more information.

This is a breaking change that makes the add() family of functions unsafe. I am very open to alternatives to this change.

The indirect effects of this change is that is makes async_io::Async::get_mut and read/write_with_mut unsafe, as well as having implications for the AsyncRead/AsyncWrite impls as well. See smol-rs/async-io#142. I would appreciate community feedback on these changes.

[notgull]

@smol-rs/admins Can anyone review this PR? Given that this change will have ecosystem wide effects, I'd like to make sure this is what we want.

[fogti]

Make sure to also review the corresponding PR for async-io. I think most of it looks good. idk if/what it would do to code further downstream to async-io and others. (API changes like the new IoSafe trait should maybe documented better /+ more prominently.

[notgull]

@taiki-e You have good judgement when it comes to this sort of thing, so I'll defer to you before I merge this.

[notgull]

I'll merge this now and we can discuss the changes in greater depth later.