Added VecInit factory methods (fill,iterate)

[anniej-sifive]

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• new feature/API

API Impact

This PR adds additional companion object factory methods to the VecInit class, such as .fill and .iterate.

A .empty method was not added because VecInit does not allow for empty hardware values at this time.

Backend Code Generation Impact

This should not change the backend code generation as it only provides different ways to fill a Vec.

Desired Merge Strategy

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

Release Notes

This PR adds additional companion object factory methods to the VecInit class, such as .fill and .iterate.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (3.2.x, 3.3.x, 3.4.x, 3.5.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?

[jackkoenig]

LGTM! Nice work!