fix(treespec): memorize ongoing repr / hash calls to resolve infinite recursion under self-referential case

[XuehaiPan]

Description

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Add a set to memorize ongoing repr / hash calls to resolve infinite recursion under self-referential case.

Motivation and Context

Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.
You can use the syntax close #15213 if this solves the issue #15213

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Fixes #81

Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

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• I have read the CONTRIBUTION guide. (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly. (required for a bug fix or a new feature)
• I have updated the documentation accordingly.
• I have reformatted the code using make format. (required)
• I have checked the code using make lint. (required)
• I have ensured make test pass. (required)

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c7c3dcd) 100.00% compared to head (916c6bd) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #82   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          425       425           
=========================================
  Hits           425       425           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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