core: use ConstantLike + HasFolderInterface for constant op results

[superlopuh]

This more closely resembles the MLIR API, where ConstantLike is a trait, and ops have optional folders.

[codecov]

Codecov Report

❌ Patch coverage is 98.03922% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.26%. Comparing base (2ec6429) to head (3b0637e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
xdsl/dialects/equivalence.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5660      +/-   ##
==========================================
- Coverage   86.26%   86.26%   -0.01%     
==========================================
  Files         403      403              
  Lines       56881    56867      -14     
  Branches     6565     6564       -1     
==========================================
- Hits        49068    49054      -14     
- Misses       6282     6283       +1     
+ Partials     1531     1530       -1     

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