fix: fix fast_substitute on subarrays of symbolic variables

[AayushSabharwal]

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[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 8.88%. Comparing base (6202fa4) to head (16df6db).
Report is 79 commits behind head on master.

Files with missing lines	Patch %	Lines
src/build_function.jl	0.00%	6 Missing ⚠️
src/variable.jl	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1423      +/-   ##
=========================================
+ Coverage    3.95%   8.88%   +4.92%     
=========================================
  Files          51      51              
  Lines        4824    4875      +51     
=========================================
+ Hits          191     433     +242     
+ Misses       4633    4442     -191     

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