Added io Module to bevy_platform_support

[bushrat011899]

Objective

Continuing no_std proliferation in Bevy is currently blocked on bevy_asset which makes extensive use of the standard library in two main areas:

• std::io
• std::path
  This is entirely sensible, since bevy_asset's primary job is to load assets from a path using filesystem IO.

However, assets can be used outside of strict IO operations. For example, users can manually add assets to an appropriate Assets<T> resource. Further, while Bevy supports std::path::Path, we have strong rules around what's considered canonical for an AssetPath; namely that platform specific behaviour (backslashes and drive letters on Windows for example) is not supported.

Since no_std support is likely to be highly disruptive in bevy_asset, this PR attempts to tackle a subset of the problem: independence from std::io::Result.

Solution

• Created a new io module in bevy_platform_support which provides Error, Result, and ErrorKind.
• When the standard library is not available, bevy_platform_support::io will fallback to an API compatible alternative.
• Switch bevy_asset to use bevy_platform_support::io::Result. Since it is std::io::Result with the std feature enabled this is purely a path change and has no impact on API or functionality.

Testing

• CI

---

Notes

• It may be desirable to actually make a more breaking change to bevy_asset and instead roll its own AssetIoError type. However, this is likely more controversial so I have opted for this as a more gradual step.

[bushrat011899]

Putting on the 0.17 milestone just to highlight I don't intend for this to land in 0.16. It could, but I'd rather not make that milestone harder to hit than it already is!