Speed Improvement: Make incoming entities Arc<Entities> like schema entities

[tomlikestorock]

Description of changes

This provides a 3x speedup based on local benchmarking with a large schema that has big action entity hierarchy relationships. Today Entities.entities is a HashMap of <EntityUID, Entity>. When from_entities brings in the entities and actions from a given schema, it currently relies on Arc::unwrap_or_clone, which frequently ends up being clone. For a large set of very interrelated Actions, this repeated clone becomes expensive.

Changing Entities.entities to be a HashMap of <EntityUID, Arc<Entity>> allows us to keep the clone cheap, and Arc'ing the incoming Entities becomes cheap as well.

Changing the interface of the HashMap means altering what create_entities_map returns, adding an implementation of TCNode for Arc<Entity>, and udpating how IntoIterator works to return values from the Arc's.

Issue #, if available

This contributes to 1285.

Checklist for requesting a review

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

• [x] A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)

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• [x] Does not update the CHANGELOG because my change does not significantly impact released code.

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• [x] Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.

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• [x] Does not require updates because my change does not impact the Cedar language specification.

[cdisselkoen]

Thanks for this work!

[tomlikestorock]

Protobuf tests updated to use Arc wrapped entities in 857509b.