Add iOS Support

[Gordon-F]

TODO:

• [x] Fix RenderPassDescriptor validation error 2020-04-05 23:24:34.518550+0300 game_ios[374:22040] validateRenderPassDescriptor, line 943: error 'renderTargetWidth (3600) must be <= minimum attachment width (1200).' validateRenderPassDescriptor:943: failed assertion 'renderTargetWidth (3600) must be <= minimum attachment width (1200).' winit issue (https://github.com/rust-windowing/winit/pull/1532)
• [x] Implement touch input handler
• [x] Rewrite README
• [x] Add aarch64-apple-ios build on CI

[Gordon-F]

@kvark @msiglreith This is very simple ios port. At least it's not crashed on device :)

I think we can improve it in future PR's.