Use new lazy serialization in FIRRTL

[jackkoenig]

This enables emission of modules that serialize to >2 GiB of .fir text.

See chipsalliance/firrtl#2554.

This has no impact on performance for most designs, it slightly improves serialization performance for designs with very large modules (due to the cost of allocating large Strings--but the JVM is very good at this so the impact is small).

Contributor Checklist

• 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
• performance improvement

API Impact

No change

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

Support serialization of Modules that require over 2 GiB of .fir text.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (Bug fix: 3.4.x, [small] API extension: 3.5.x, API modification or big change: 3.6.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.