Miscellaneous CompiledSimulation and Simulation cleanup, for 8% speedup (pyrtlnet CompiledSimulation startup)

[fdxmw]

• Refactor to remove redundant code.
• Use f-strings instead of .format.
• Use _limb_name (formerly _getarglimb) consistently.
• Define a dataclass for _build_concat's pieces so the pieces have names.
• Rename a lot of variables to improve readability.
• Add comments, type annotations, and assertions.

[codecov]

Codecov Report

❌ Patch coverage is 99.46524% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.9%. Comparing base (baaf61f) to head (aa02467).

Files with missing lines	Patch %	Lines
pyrtl/compilesim.py	99.5%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           development    #497     +/-   ##
=============================================
+ Coverage         92.3%   93.9%   +1.6%     
=============================================
  Files               30      30             
  Lines             7488    7138    -350     
=============================================
- Hits              6905    6696    -209     
+ Misses             583     442    -141     

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[fdxmw]

The coverage report shows that some pretty important lines of CompiledSimulation code are not covered by tests, mostly limb boundary checks. New tests that were meant to be added to this PR found a bug, and I believe I've fixed that bug in #498, so I'm planning to merge #498 first, then rebase this PR.