Fix merging for SomeSymBV

[lsrcz]

We should allow SomeSymBV with different bit size coexists in a union. This pull request supports this.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.11 ⚠️

Comparison is base (71ed075) 32.50% compared to head (ad72121) 32.39%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
- Coverage   32.50%   32.39%   -0.11%     
==========================================
  Files          49       49              
  Lines        5006     5016      +10     
  Branches      245      247       +2     
==========================================
- Hits         1627     1625       -2     
- Misses       3134     3144      +10     
- Partials      245      247       +2     

Impacted Files	Coverage Δ
src/Grisette/Core/Control/Monad/UnionM.hs	10.82% <ø> (+0.11%)	⬆️
src/Grisette/Core/Data/Class/Mergeable.hs	15.62% <0.00%> (-0.80%)	⬇️
src/Grisette/Core/Data/Class/SimpleMergeable.hs	6.80% <ø> (+0.09%)	⬆️

... and 2 files with indirect coverage changes

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