Introducing the two stacking functions HStack + VStack

[kwesiRutledge]

Also introduced the method ConcretizeExpression which helps make this function intuitive.

[codecov]

Codecov Report

Attention: Patch coverage is 80.67227% with 23 lines in your changes missing coverage. Please review.

Project coverage is 85.41%. Comparing base (d40667a) to head (f8fae16).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
symbolic/expression.go	70.42%	20 Missing and 1 partial ⚠️
symbolic/monomial.go	0.00%	1 Missing ⚠️
symbolic/variable_matrix.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
- Coverage   86.17%   85.41%   -0.77%     
==========================================
  Files          38       38              
  Lines        5462     5676     +214     
==========================================
+ Hits         4707     4848     +141     
- Misses        700      763      +63     
- Partials       55       65      +10     

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