Tracking Issue: `no_std` for `bevy_asset`

[bushrat011899]

Motivation

bevy_asset is a key crate within Bevy that makes heavy usage of std, making it challenging to add no_std support to. This is a tracking issue to document what I anticipate will be a lengthy process.

End Goal

• To support some subset of bevy_asset on no_std targets, such as wasm32v1-none

Tasks

Below are some particular tasks which will need to be accomplished prior to no_std support. They are in no particular order.

• Remove or make optional parking_lot
• Add no_std support to ron
• Handle the following incompatible dependencies:
  • async-broadcast
  • async-fs
  • crossbeam-channel
  • futures-io
• Make AssetPath independent of std::path::Path (Cart has previously expressed that AssetPath should always be valid UTF-8 and only use / as its path separator, implying str is a suitable backend)
• Make bevy_asset independent of std::io::Error.
• Feature-gate built-in filesystem IO support (std::fs)

[bushrat011899]

Can potentially use a technique similar to ciborium-io to provide an intermediary between std::io and bevy_asset::io which adds no_std support. The question becomes whether:

1. Use ciborium-io itself
2. Add a custom alternative to bevy_platform that is as-close-as-possible to std::io
3. Flesh out AssetReader, AssetWriter, Reader, Writer, etc. in bevy_asset and apply the technique at that higher level

Since bevy_asset requires std::io and futures_io to fully define its IO traits, we'd need to do this across a fairly wide API surface area (I estimate a couple thousand LOC plus tests). Definitely needs consideration before choosing a path.