hdl._mem: implement MemoryData._Row from RFC 62.

[wanda-phi]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 73.64865% with 39 lines in your changes are missing coverage. Please review.

Project coverage is 89.63%. Comparing base (f71bee4) to head (4a56232).
Report is 1 commits behind head on main.

Files	Patch %	Lines
amaranth/sim/pysim.py	32.00%	14 Missing and 3 partials ⚠️
amaranth/hdl/_dsl.py	85.54%	8 Missing and 4 partials ⚠️
amaranth/sim/_pyeval.py	61.53%	2 Missing and 3 partials ⚠️
amaranth/hdl/_mem.py	86.36%	2 Missing and 1 partial ⚠️
amaranth/sim/core.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1271      +/-   ##
==========================================
- Coverage   89.88%   89.63%   -0.26%     
==========================================
  Files          44       44              
  Lines       10325    10417      +92     
  Branches     2510     2548      +38     
==========================================
+ Hits         9281     9337      +56     
- Misses        844      871      +27     
- Partials      200      209       +9     

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[whitequark]

Thanks!