perf: cppgc inheritance rework

[nathanwhit]

A lot of the new lines are docs, see the readme for a synopsis. I've been successfully using this in my R&D branch for a few weeks now with no issues. Benchmarks (don't have numbers on hand atm) show a notable improvement in new object creation and op dispatch when there is inheritance

[codecov-commenter]

Codecov Report

❌ Patch coverage is 56.13682% with 218 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.61%. Comparing base (0c7f83e) to head (76baf63).
⚠️ Report is 430 commits behind head on main.

Files with missing lines	Patch %	Lines
ops/op2/mod.rs	21.95%	64 Missing ⚠️
core/cppgc.rs	57.95%	37 Missing ⚠️
testing/checkin/runner/ops.rs	41.07%	33 Missing ⚠️
ops/cppgc.rs	77.27%	30 Missing ⚠️
ops/op2/signature.rs	41.66%	21 Missing ⚠️
ops/op2/config.rs	42.10%	11 Missing ⚠️
ops/op2/dispatch_fast.rs	60.00%	10 Missing ⚠️
ops/op2/dispatch_slow.rs	66.66%	6 Missing ⚠️
ops/op2/object_wrap.rs	85.29%	5 Missing ⚠️
ops/op2/dispatch_async.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1300      +/-   ##
==========================================
- Coverage   81.43%   77.61%   -3.83%     
==========================================
  Files          97      105       +8     
  Lines       23877    26728    +2851     
==========================================
+ Hits        19445    20744    +1299     
- Misses       4432     5984    +1552     

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