Description of changes

This change adds Expression::new_duration, Expression::new_datetime, RestrictedExpression::new_duration and RestrictedExpression::new_datetime methods to the public API. These are analogous to the existing new_ip, etc. methods for constructing expressions with extension function values, but for the new datetime and duration types from RFC 80.

It also corrects some incorrect uses of assert_matches! in unit tests for the old new_ip and new_decimal functions.

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):

• 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.