Support iOS and Android

[eycorsican]

I made some changes to this crate and it works on iOS: eycorsican@b5cdf61

The idea is that on iOS and Android, we can create and configure a TUN interface in native code, then pass the file descriptor to rust-tun to create an async interface for reading/writing.

[meh]

Could you make a pull request so I can review it?

Also could you run cargo fmt in a separate commit so it's easier to review please?

[eycorsican]

I can make a PR for iOS, would you mind a PR with rustfmt apply to the whole project?

[meh]

You can put them both in the same PR as long as they're separate commits 🐼