Use circuit name hashCode for circuit anno hashCode

[jackkoenig]

It is unclear if having more than one circuit annotation even works. If
it does, it is implausible that they would have the same circuit name
since that would correspond to a namespace collision.

Looking at flamegraphs, FirrtlCircuitAnnotation.hashCode always comes up. Creating a HashMap with different Circuits that nevertheless have the same name is extremely uncommon so collisions are not a concern. Include productPrefix so that this doesn't collide with other types that use a similar strategy and hash the same String.

This speeds up anything using Stage/Phase with FIRRTL Circuits which includes Chisel

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

• performance improvement

API Impact

No impact

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

Improve performance of FirrtlCircuitAnnotation.hashCode. This speeds up all uses of Stage/Phase.

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?

[ekiwi]

Seems reasonable 🚢