Restore and deprecate Chisel.Driver

[jackkoenig]

It shouldn't be removed until we also remove underlying chisel3.Driver

Contributor Checklist

• [NA] 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

• bug fix

API Impact

Chisel.Driver will continue to work, for the time being

Backend Code Generation Impact

None

Desired Merge Strategy

• Squash

Release Notes

Restore Chisel.Driver

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (3.2.x, 3.3.x, 3.4.0, 3.5.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?

[edwardcwang]

Does this unbreak something broken?

[jackkoenig]

Does this unbreak something broken?

Yes, code within SiFive that is fixed by switching to chisel3.Driver which feels wrong

[seldridge]

More info: chisel3.Driver proved very, very difficult to actually remove due to usages in iotesters and related projects. It then seems odd to remove Chisel.Driver while keeping chisel3.Driver around. So, the plan is to keep them both in place (and deprecated), and to remove them both at the same time.