Deprecate all mutable methods on RenameMap and add MutableRenameMap

[jackkoenig]

• Add renamemap.MutableRenameMap which includes these methods without
  deprecation
• Deprecate Stringly typed RenameMap APIs which were accidentally
  undeprecated a while ago

Much friendlier version of #2318

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• [NA] Did you update the FIRRTL spec to include every new feature/behavior?
• 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 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

• code refactoring
• code cleanup

API Impact

This deprecates all mutable methods on RenameMap, the exact same functionality is maintained on the new firrtl.renamemap.MutableRenameMap. RenameMap is also now a [sealed] trait which will enable alternative (optimized) implementations in the future.

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

Deprecate all mutable methods on RenameMap. For constructing RenameMaps, use firrtl.renamemap.MutableRenameMap.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (1.2.x, 1.3.0, 1.4.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?