Printables for verification preconditions

[adkian-sifive]

Continuation of #2527

Additions include unit test for assume and Scaladoc group for internal macros

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

API Impact

Backend Code Generation Impact

Desired Merge Strategy

Release Notes

Reviewer Checklist (only modified by reviewer)

• [ ] Did you add the appropriate labels?
• [ ] Did you mark the proper milestone (Bug fix: 3.4.x, [small] API extension: 3.5.x, API modification or big change: 3.6.0)?
• [ ] 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.
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[jackkoenig]

The ScalaDoc still leaves much to be desired (see screenshot below). Compare it to how things are grouped on UInt (https://javadoc.io/doc/edu.berkeley.cs/chisel3_2.13/latest/chisel3/UInt.html). Can we:

1. Make sure every _applyMacro... function is in the VerifPrintMacros group?
2. Ensure the warning description of VerifPrintMacros shows up?
3. Put VerifPrintMacros below the "Ungrouped" group?

[jackkoenig]

ScalaDoc looks great, thanks!