hdl._mem: add MemoryData class.

[wanda-phi]

This implements half of RFC 62. The MemoryData._Row class will be implemented later, as a follow-up.

[codecov]

Codecov Report

Attention: Patch coverage is 91.11111% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 89.78%. Comparing base (1deaf70) to head (bc04b8f).

Files	Patch %	Lines
amaranth/sim/pysim.py	62.85%	10 Missing and 3 partials ⚠️
amaranth/hdl/_mem.py	97.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1256      +/-   ##
==========================================
- Coverage   89.80%   89.78%   -0.03%     
==========================================
  Files          43       43              
  Lines       10008    10055      +47     
  Branches     2426     2439      +13     
==========================================
+ Hits         8988     9028      +40     
- Misses        823      831       +8     
+ Partials      197      196       -1     

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[whitequark]

Thanks!