fix: recursively collect and apply parameters in closures

[divarvel]

Datalog parameters are managed lazily: upon building a rule, referenced parameter are collected in a map. Setting them only updates the map, and the actual AST is only updated as needed (such as in Display impls, or when converting to datalog data structures).

Expressions used to be linear, but closures introduced the possibility of recursive expressions, so the code inside closure bodies was ignored both when collecting parameters, and when applying them.

This caused an issue because some parts of the code assume that unbound parameters are handled beforehand, and panic upon encountering them.

[codspeed-hq]

CodSpeed Performance Report

Merging #229 will not alter performance

_{Comparing fix-param-substitution-in-closures (35edbd8) with v5 (911ebe4)}

Summary

✅ 12 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.48%. Comparing base (911ebe4) to head (35edbd8).
Report is 2 commits behind head on v5.

Files with missing lines	Patch %	Lines
biscuit-auth/src/token/builder.rs	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v5     #229      +/-   ##
==========================================
+ Coverage   68.42%   68.48%   +0.05%     
==========================================
  Files          25       25              
  Lines        5605     5625      +20     
==========================================
+ Hits         3835     3852      +17     
- Misses       1770     1773       +3     

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[divarvel]

capi support will be extracted to a dedicated crate, so we can ignore the failing build step.