Add hook to invoke hardware generators at the end of elaboration.

[mikeurbach]

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?
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Type of Improvement

• Feature (or new API)

This adds a new API to invoke generators late, within the scope of a Module that has just been elaborated. It is intended as a power user API, and should not normally be needed. However, it does unlock new functionality in some cases where it would otherwise be impossible.

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

Release Notes

Reviewer Checklist (only modified by reviewer)

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[mikeurbach]

@jackkoenig @mwachs5 @seldridge we discussed an API like this a while back. Here is my take after that discussion, please let me know what you think.

[mikeurbach]

I pushed up some documentation and new tests, but the implementation remains the same. Is there anything else I should add in terms documentation or tests? Is this the implementation we want? I believe we discussed a few possible APIs in this vein that @jackkoenig was interested in, so I want to make sure the current implementation is actually one of the ones we want.

[mikeurbach]

Also tagging @mwachs5 @azidar @seldridge in case you all have any thoughts on this one.

[mwachs5]

thanks for the tests! lgtm

[jackkoenig]

This API is likely to change a lot in the near future, but it's useful for the time being so let's go ahead and