Add implementation of std::fmt::Display for Eid

[shaobo-he-aws]

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

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

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

• 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.

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.

[cdisselkoen]

This was intentionally removed in 4.0.0, see changelog. The reasoning was that it's not clear whether the default should be escaped or unescaped. Has something changed that we want this now?

[shaobo-he-aws]

Nothing changed since. I'll close this PR.