Add lazy calculation of constraint dsu

[dim8art]

Summary:

Checklist:

• The PR addresses a single issue. If it can be divided into multiple independent PRs, please do so.
• The PR is divided into a logical sequence of commits OR a single commit is sufficient.
• There are no unnecessary commits (e.g. commits fixing issues in a previous commit in the same PR).
• Each commit has a meaningful message documenting what it does.
• All messages added to the codebase, all comments, as well as commit messages are spellchecked.
• The code is commented OR not applicable/necessary.
• The patch is formatted via clang-format OR not applicable (if explicitly overridden leave unchecked and explain).
• There are test cases for the code you added or modified OR no such test cases are required.

[codecov-commenter]

Codecov Report

Merging #156 (3fb1287) into main (c1648ab) will increase coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head 3fb1287 differs from pull request most recent head f18c081. Consider uploading reports for the commit f18c081 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
+ Coverage   66.99%   67.04%   +0.05%     
==========================================
  Files         221      221              
  Lines       30046    30086      +40     
  Branches     6600     6603       +3     
==========================================
+ Hits        20130    20172      +42     
+ Misses       7403     7402       -1     
+ Partials     2513     2512       -1     

Files	Coverage Δ
include/klee/Expr/AlphaBuilder.h	100.00% <ø> (ø)
lib/Core/ExecutionState.cpp	74.10% <100.00%> (+0.09%)	⬆️
lib/Expr/Constraints.cpp	84.61% <100.00%> (+0.61%)	⬆️
lib/Expr/IndependentConstraintSetUnion.cpp	96.52% <100.00%> (+1.06%)	⬆️

... and 1 file with indirect coverage changes