Implement kani::Arbitrary for Box<T>

[adpaco-aws]

Description of changes:

This PR implements Arbitrary for Box<T>. This is required for structs that #[derive(kani::Arbitrary)] and have members with Box<T> types.

Resolved issues:

Resolves #2403

Call-outs:

• I haven't thought too much about the best way to test these changes, so I'm open to suggestions.
• The same-purpose tests added in Implement Arbitrary for PhantomData and PhantomPinned #2225 were in the ui folder. I moved them to expected because that seems to be a better place (no UI testing is done in those tests).

Testing:

• How is this change tested? Existing tests + new one.

• 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.

[zhassan-aws]

I'm concerned about the soundness of the Arbitrary implementation. We need to make sure the implementation covers all possible states a box can be in. I'm a bit unclear on whether this is the case with the current implementation.