compiler: Concretize SubDimensions to same object across repeated calls to concretize_subdims

[ZoeLeibowitz]

Ensure that SubDimensions are consistently concretized to the same object across multiple calls to the function. This is important when using rcompile on equations with SubDimensions.

TODO: Enhance SymbolRegistry by making it rebuildable, enabling operations such as rcompile on specific subsets.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.29%. Comparing base (71e7eda) to head (6835b2b).

Files with missing lines	Patch %	Lines
tests/test_builtins.py	50.00%	2 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2509   +/-   ##
=======================================
  Coverage   87.28%   87.29%           
=======================================
  Files         238      238           
  Lines       45703    45716   +13     
  Branches     4057     4057           
=======================================
+ Hits        39892    39906   +14     
- Misses       5126     5127    +1     
+ Partials      685      683    -2     

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[FabioLuporini]

Not completely sure carrying around an extra mapper is the best way but don't have a better option top of my head without lots of rework

We discussed this in yesterday's meeting, @mloubout .

My suggestion was to rather use sregistry. And, in fact, we could (should) use sregistry also in place of that mapper we are currently carrying along throughout the whole pass