lib.io: Implement *Buffer from RFC 55.

[wanda-phi]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 95.81395% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 89.78%. Comparing base (d6bf47d) to head (1e88d42).
Report is 5 commits behind head on main.

Files	Patch %	Lines
amaranth/lib/io.py	95.81%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1215      +/-   ##
==========================================
+ Coverage   89.62%   89.78%   +0.16%     
==========================================
  Files          43       43              
  Lines        9722     9929     +207     
  Branches     2332     2396      +64     
==========================================
+ Hits         8713     8915     +202     
- Misses        819      823       +4     
- Partials      190      191       +1     

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[whitequark]

LGTM, thanks!

The docstrings are intentionally not linked from docs/stdlib/io.rst for the time being; I will take care of that later.