Deprecate CompilerAnnotation

[seldridge]

This is a follow-on to #1835 to fully deprecate CompilerAnnotation.

This does a couple of things:

1. The CompilerAnnotation$ companion object is changed to emit RunFirrtlTransformAnnotations instead of CompilerAnnotations. In effect, when you do -X verilog, you now get a RunFirrtlTransformAnnotation(new VerilogEmitter) as opposed to a CompilerAnnotation(new VerilogCompiler).

2. This adds a ConvertCompilerAnnotation phase which is doing an explicit conversion of a CompilerAnnotation to a RunFirrtlTransformAnnotation. This differs from stage: allow a RunFirrtlTransformAnnotation(_:Emitter) annotation to be used in place of a CompilerAnnotation #1835 which relaxed support so that a RunFirrtlTransformAnnotation(_ <: Emitter) could substitute for a CompilerAnnotation. This PR goes one step further and forces internal phases to only understand transforms.

3. Better checks and warnings are now provided inside ConvertCompilerAnnotation. The check of a single compiler is migrated from Checks here. Additionally, if a CompilerAnnotation is converted, a verbose warning is printed telling you exactly how to replace your transform, e.g., you now get:

CompilerAnnotation is deprecated since FIRRTL 1.4.0. Please use 'RunFirrtlTransformAnnotation(new firrtl.HighFirrtlEmitter)' instead.

Note that you should only now get a warning if you actually use a CompilerAnnotation. Any use of -X will not produce an error.

Contributor Checklist

• [nope] Did you add Scaladoc to every public function/method?
• [n/a] 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

• bug fix

• code refactoring

API Impact

None. Removes a warning that the user couldn't do anything to fix.

Backend Code Generation Impact

None.

Desired Merge Strategy

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

Release Notes

None.

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?

[jackkoenig]

This fixes the annoying deprecation warning when running (new ChiselStage).emitChirrtl (or whatever) and looks good. A few minor questions but the overall PR looks great.

[ekiwi]

Did you make sure that specifying only -X without -E works? What stage will add the EmitCircuitAnnotation?

[seldridge]

@ekiwi: firrtl.stage.phases.AddImplicitEmitter is what is taking care of this. Fortunately, your edits in your original PR are already handling this case nicely.

I added a test to FirrtlMainSpec to hit this in a follow-up commit.

[ekiwi]