Optimize entity type representation for strict validation

[john-h-kastner-aws]

Description of changes

In strict validation, the default and only non-experimental validation mode, we never have non-singleton entity types, but still represent them as a set of entity types. This PR changes the representation from BTreeSet<EntityType> to NonEmpty<EntityType> to avoid allocating a set for every entity type in strict validation. The single-element case gets stored inline, but in permissive validation we will still heap allocate for the rest. This is possibly less efficient for permissive validation, but should be negligible or even faster for reasonable polices that don't construct massive unions of entities.

Measured on strict validation as ~1.2x faster on some simple policies.

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.)

[github-actions]

Coverage Report

Head Commit: 23b945ff58170605223cc6eec5f48f15bd8f808b

Base Commit: 15625054b79fdbf1eac7129cafbc5bad97966094

Download the full coverage report.

Coverage of Added or Modified Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 96.55%

Status: PASSED ✅

Details

File	Status	Covered	Coverage	Missed Lines
cedar-policy-core/src/validator/types.rs	🟢	28/29	96.55%	842

Coverage of All Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 88.39%

Status: PASSED ✅

Details

Package	Status	Covered	Coverage	Base Coverage
cedar-language-server	🟢	4722/5102	92.55%	--
cedar-policy	🟢	4869/5971	81.54%	--
cedar-policy-cli	🟡	1294/1669	77.53%	--
cedar-policy-core	🟢	24603/27761	88.62%	--
cedar-policy-formatter	🟢	914/1088	84.01%	--
cedar-policy-symcc	🟢	6931/7405	93.60%	--
cedar-wasm	🔴	0/28	0.00%	--