Fix entity manifests bug caused by aliasing between entity literal and var

[john-h-kastner-aws]

Description of changes

Given a policy like

    permit(
      principal is a in a::"",
      action in [Action::"action"],
      resource
    ) when {
      (true && ((a::"" has "A") == false)) && (a::"" in [a::""])
    };

and a request where the principal is a::"", the slicing algorithm failed to select take the attribute A because it did not correctly account for principal and a::"" referencing the same entity.

Issue #, if available

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A breaking change requiring a major version bump to cedar-policy (e.g., changes to the signature of an existing API).
• A backwards-compatible change requiring a minor version bump to cedar-policy (e.g., addition of a new API).
• A bug fix or other functionality change requiring a patch to cedar-policy.
• A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)
• A change (breaking or otherwise) that only impacts unreleased or experimental code.

I confirm that this PR (choose one, and delete the other options):

• Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).
• Does not update the CHANGELOG because my change does not significantly impact released code.

I confirm that cedar-spec (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.
• Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-spec, and how you have tested that your updates are correct.)
• Requires updates, but I do not plan to make them in the near future. (Make sure that your changes are hidden behind a feature flag to mark them as experimental.)
• I'm not sure how my change impacts cedar-spec. (Post your PR anyways, and we'll discuss in the comments.)

I confirm that docs.cedarpolicy.com (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar language specification.
• Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-docs. PRs should be targeted at a staging-X.Y branch, not main.)
• I'm not sure how my change impacts the documentation. (Post your PR anyways, and we'll discuss in the comments.)

[cdisselkoen]

consider debug_assert instead of assert, since these might have nontrivial runtime cost?

[john-h-kastner-aws]

I'm actually closer to lifting this to an Err result for invariant violation