Fix std overrides in crates where std is used as a feature

[zhassan-aws]

Description of changes:

Kani's std overrides relied on creating an alias for core::assert up front to use for error checking in various macros (assert, panic, etc.). Creating the alias upfront doesn't work properly in crates with no_std where std is used as a feature (see #2187). This PR updates the std overrides to use core::assert directly when an std feature is enabled.

Resolved issues:

Resolves #2187

Related RFC:

Optional #ISSUE-NUMBER.

Call-outs:

To avoid code duplication, I tried creating a macro that calls __kani__workaround_core_assert under not(std) and core::assert otherwise. However, I ran into the same error reported in #2187.

Testing:

• How is this change tested? Added one test

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[danielsn]

Should we also have a test where the assertion fails?

[zhassan-aws]

I don't think this is necessary, since we're mainly checking that compilation goes through. There are many other tests that check for failing assertions.

[celinval]

I'm worried that this is getting out of control. We should investigate a more robust solution.

Thanks for the quick fix though.