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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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[Dirbaio]

thanks!