Operator for measuring sizes of unions

[lsrcz]

This pull request introduces a utility function for measuring union sizes.

>>> unionSize (single True)
1
>>> unionSize (mrgIf "a" (single 1) (single 2) :: UnionM Integer)
2
>>> unionSize (choose [1..7] "a" :: UnionM Integer)
7

[codecov]

Codecov Report

Base: 32.32% // Head: 32.63% // Increases project coverage by +0.30% 🎉

Coverage data is based on head (b526752) compared to base (3638ec5).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   32.32%   32.63%   +0.30%     
==========================================
  Files          48       48              
  Lines        4523     4526       +3     
  Branches      220      216       -4     
==========================================
+ Hits         1462     1477      +15     
+ Misses       2841     2833       -8     
+ Partials      220      216       -4     

Impacted Files	Coverage Δ
src/Grisette/Core/Control/Monad/UnionM.hs	11.17% <100.00%> (+1.98%)	⬆️
src/Grisette/Core/Data/Class/Mergeable.hs	16.19% <0.00%> (ø)
.../Grisette/IR/SymPrim/Data/Prim/PartialEval/Bool.hs	86.66% <0.00%> (+0.44%)	⬆️
src/Grisette/Core/Data/Class/SimpleMergeable.hs	6.71% <0.00%> (+0.67%)	⬆️
src/Grisette/Core/Data/Union.hs	46.23% <0.00%> (+1.07%)	⬆️
src/Grisette/Core/Data/Class/GenSym.hs	5.41% <0.00%> (+2.06%)	⬆️

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