back.rtlil: refactor to use intermediate structures.

[wanda-phi]

Fixes #1100.

[codecov]

Codecov Report

Attention: Patch coverage is 94.68439% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 90.85%. Comparing base (262e24b) to head (4aed0b3).

Files	Patch %	Lines
amaranth/back/rtlil.py	94.68%	14 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1278      +/-   ##
==========================================
+ Coverage   89.52%   90.85%   +1.32%     
==========================================
  Files          44       44              
  Lines       10480    10558      +78     
  Branches     2560     2575      +15     
==========================================
+ Hits         9382     9592     +210     
+ Misses        891      776     -115     
+ Partials      207      190      -17     

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[whitequark]

Thanks!