Implement RFC 10: Move Repl to Value.replicate

[ghost]

Implements RFC 10; fixes #770.

• I've marked Repl as for removal in 0.5 with a comment, per the changelog text:

  While code that uses the features listed as deprecated below will work in Amaranth 0.4, they will be removed in the next version.

• I've left the "iffy cast" warnings in for the Repl wrapper.

• The regression tests for #325 (diff) now reduce to the same AST, so I removed the replicate one, as they no longer test different paths.

[codecov]

Codecov Report

Merging #825 (2a1282b) into main (b1cce87) will decrease coverage by 0.04%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main     #825      +/-   ##
==========================================
- Coverage   82.11%   82.08%   -0.04%     
==========================================
  Files          50       50              
  Lines        6873     6839      -34     
  Branches     1673     1667       -6     
==========================================
- Hits         5644     5614      -30     
+ Misses       1034     1031       -3     
+ Partials      195      194       -1     

Impacted Files	Coverage Δ
amaranth/back/rtlil.py	78.97% <ø> (-0.11%)	⬇️
amaranth/hdl/mem.py	98.16% <ø> (ø)
amaranth/hdl/xfrm.py	96.19% <ø> (-0.06%)	⬇️
amaranth/sim/_pyrtl.py	95.42% <ø> (-0.16%)	⬇️
amaranth/compat/fhdl/structure.py	67.71% <50.00%> (ø)
amaranth/hdl/ast.py	89.83% <100.00%> (+0.29%)	⬆️

... and 1 file with indirect coverage changes

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[whitequark]

LGTM. Thank you for this excellent PR! It's not very often that a first-time contributor ticks all the boxes on their very first PR right away.

[ghost]

It was my pleasure! This was the perfect intro for making sure I did actually catch everything outside the functionality change itself. 😊