Remove Implicit std Prelude from no_std Crates

[bushrat011899]

Background

In no_std compatible crates, there is often an std feature which will allow access to the standard library. Currently, with the std feature enabled, the std::prelude is implicitly imported in all modules. With the feature disabled, instead the core::prelude is implicitly imported. This creates a subtle and pervasive issue where alloc items may be implicitly included (if std is enabled), or must be explicitly included (if std is not enabled).

Objective

• Make the implicit imports for no_std crates consistent regardless of what features are/not enabled.

Solution

• Replace the cfg_attr "double negative" no_std attribute with conditional compilation to include std as an external crate.

// Before
#![cfg_attr(not(feature = "std"), no_std)]

// After
#![no_std]

#[cfg(feature = "std")]
extern crate std;

• Fix imports that are currently broken but are only now visible with the above fix.

Testing

• CI

Notes

I had previously used the "double negative" version of no_std based on general consensus that it was "cleaner" within the Rust embedded community. However, this implicit prelude issue likely was considered when forming this consensus. I believe the reason why is the items most affected by this issue are provided by the alloc crate, which is rarely used within embedded but extensively used within Bevy.

[alice-i-cecile]

Very reasonable diff: I like that this is more explicit and less likely to cause weird CI problems for contributors. Ping me when CI is behaving :)

[alice-i-cecile]

Ooh, you've unlocked the next, harder level of CI.

[bushrat011899]

Ok @alice-i-cecile I'm ready for the next attempt at merging this thing!

[bushrat011899]

Ok one last CI action is failing in the merge queue. Back to the IDE...

[bushrat011899]

Marking as ready again. I believe I have resolved all CI issues, but the run-examples-on-wasm validation task is unclear to me. I fixed a warning that task highlighted, but I don't think that was the cause of the timeout or panic.