Make ToSym for Union merges again

[lsrcz]

This pull request adds the Mergeable superclass to ToSym. Also adds Mergeable constraint to liftToSym. This enables us to merge the resulting Union when we do toSym from Union to Union.

[codecov]

Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Project coverage is 54.54%. Comparing base (d3e67e3) to head (b607c7d).
Report is 3 commits behind head on main.

Files	Patch %	Lines
src/Grisette/Internal/Core/Data/Class/ToSym.hs	44.44%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
+ Coverage   54.50%   54.54%   +0.04%     
==========================================
  Files         117      117              
  Lines        9620     9625       +5     
  Branches      900      898       -2     
==========================================
+ Hits         5243     5250       +7     
  Misses       3477     3477              
+ Partials      900      898       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.