Tweak constructor to avoid re-building attrs Map

[john-h-kastner-aws]

Description of changes

Fix #540

Avoid building a map twice in some cases by having the constructor take an impl IntoIterator<Item = (SmolStr, PartialValue)> instead of HashMap. There also already exists new_with_attr_partial_value_serialized_as_expr which can be called to avoid re-building when the attrs are already a BTreeMap.

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.
• [x] 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.

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

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