[Formal] Expose Btor2 target

[dobios]

This PR exposes the btor2 target in Chisel. This is done through an emitBtor2 method in ChiselStage.
This currently depends on a CIRCT PR to support AssertProperty.

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 request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Feature (or new API)

Desired Merge Strategy

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

Release Notes

• [Formal] Allow for Chisel to be compiled to btor2 for Bounded Model Checking using ChiselStage.emitBtor2.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: dobios / name: Amelia Dobis (929d921, 141dcd4, 3cb68c5, 73a37c8, 3053b62, 728862c, 4ea5428, 505a293, 4220dca, 424817b, c31176f)
• ✅ login: jackkoenig / name: Jack Koenig (9d3efcd)

[sequencer]

Nice! can't wait to use this feature! My repo haven't go though the formal pass since I upgrade to chisel5.

[dobios]

The blocking PR has been merged in and should be included in the next firtool release, which would make this work.

[dobios]

The new firtool release now supports everything that is needed for this PR!

[jackkoenig]

Looks great! Awesome work.