Aggregate literals now materialize const wires

[trilorez]

This should be fairly innocuous change that was originally blocked by firtool's lack of const parsing support. This is a prerequisite for the more extensive const updates I've been working on.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
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Type of Improvement

• Internal or build-related (includes code refactoring/cleanup)

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference).

Release Notes

Aggregate literals now materialize const wires. This will facilitate further support for const in operations and the type system in the future.

Reviewer Checklist (only modified by reviewer)

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[jackkoenig]

LGTM!