Add MiMa binary compatibility checking to 3.4.x

[jackkoenig]

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?
• [NA] 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 test

API Impact

Enforces the API

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

None

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?

[jackkoenig]

@chick @ducky64 can one of you protect 3.4.x? I changed the rules in firrtl to be 1.*.x so all stable branches will be protected.

[ducky64]

Changed the protection rule from 3.2.x to 3.*.x (unless there was a reason 3.0/1/3.x were intentionally not protected?).

[ducky64]

Looks sane, except possibly the comment about changing the environment variables.

For anyone else looking at this PR: the MiMa README was a useful resource in understanding this

[ducky64]

Is this needed to support MiMa or did this bleed over from another change?

[jackkoenig]

Ah yes, sorry those are just used for the testing: https://github.com/freechipsproject/chisel3/blob/03e6c2ba149ac611d1e2329c4502fed0ccea48fe/.circleci/config.yml#L66

The FIRRTL and Treadle variables are needed in case we make source incompatible changes to those projects on master. I don't know why CHISEL_REV exists, but I did a diff with what's on 3.3.x to determine what to change in this file.