Add minimum support for iOS

[Chilledheart]

Below is my usage of smoltcp on iOS:

• https://github.com/Chilledheart/tun2proxy/blob/master/src/context_interface.rs (replacing the raw socket)
• https://github.com/Chilledheart/yass/blob/develop/src/ios/extensions/tun2proxy.mm (hooks for iOS's packet tunnel side)
• https://github.com/Chilledheart/tun2proxy/blob/master/src/ios.rs (hooks for smoltcp side)

Tested on iOS 17 and iOS on ARM macOS.

The minimum requirement for smoltcp is making it compile when targeting iOS, so there is the patch.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d185a37) 79.82% compared to head (ccfff8b) 79.91%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #896 +/- ## ========================================== + Coverage 79.82% 79.91% +0.08% ========================================== Files 80 80 Lines 28053 28232 +179 ========================================== + Hits 22394 22561 +167 - Misses 5659 5671 +12 ```

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