Level validation Rust implementation (RFC 76)

[andrewmwells-amazon]

Description of changes

Rust implementation of Level validation (RFC 76: cedar-policy/rfcs#76)

@aaronjeline is working on Lean changes.

Example error messages:

Issue #, if available

RFC 76 (cedar-policy/rfcs#76)

Checklist for requesting a review

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

• A backwards-compatible change requiring a minor version bump to cedar-policy (e.g., addition of a new API).

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).

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

• 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.)

[aaronjeline]

This is slightly different the lean impl. In the lean impl Level 0 is the level that can't be de-referenced. I think they should align on that. Otherwise, looks good. Very different algorithm to the lean, but I think you said that was so you can get better error messages.

[andrewmwells-amazon]

This is slightly different the lean impl. In the lean impl Level 0 is the level that can't be de-referenced. I think they should align on that.

That's my understanding too. Each node's level gives the number of allowable dereferences. 0 means no dereferences are allowed. A negative number means we've already done more dereferences than allowed. None means infinite dereferences allowed. Is there a place where I'm not following this?