core: Implement speculability trait and helper.

[PapyChacal]

It is way simpler than side effects, as an op is either speculatable or is not!

Similarly to what was decided on side effects, I only changed that MLIR has an extra return value signifying "recursively speculatable", where here it is handled by an implementation of the trait, rather than in user code.
This prevents some amount of ninja-coding in said user code, but also a lot of ninja headaches.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.90%. Comparing base (d625084) to head (322a364).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3102   +/-   ##
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  Coverage   89.89%   89.90%           
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  Files         419      419           
  Lines       53034    53068   +34     
  Branches     8223     8226    +3     
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+ Hits        47677    47711   +34     
  Misses       4029     4029           
  Partials     1328     1328           

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[superlopuh]

This is great! Seems like it's missing tests ;)