Make computation of widest union member widely available

[tautschnig]

This functionality is of use beyond byte-operator lowering.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5746 (4acaa86) into develop (bc6b109) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #5746      +/-   ##
===========================================
- Coverage    69.55%   69.55%   -0.01%     
===========================================
  Files         1243     1243              
  Lines       100721   100719       -2     
===========================================
- Hits         70054    70052       -2     
  Misses       30667    30667              

Flag	Coverage Δ
cproversmt2	43.29% <94.11%> (-0.01%)	⬇️
regression	66.46% <82.35%> (+<0.01%)	⬆️
unit	32.22% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/util/std_types.h	94.46% <ø> (ø)
src/solvers/lowering/byte_operators.cpp	83.76% <100.00%> (-0.21%)	⬇️
src/util/pointer_offset_size.cpp	94.80% <100.00%> (-0.04%)	⬇️
src/util/std_types.cpp	90.81% <100.00%> (+1.04%)	⬆️

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