Refine instances for ITEOp, SimpleMergeable and Mergeable, rename UnionMergeable1 to SymBranching

[lsrcz]

This pull request:

• refines instances for ITEOp, SimpleMergeable and Mergeable,
• renames UnionMergeable1 to SymBranching
• adds support for unified SimpleMergeable and Mergeable.

[codecov]

Codecov Report

Attention: Patch coverage is 42.48826% with 245 lines in your changes missing coverage. Please review.

Project coverage is 53.32%. Comparing base (d7854bb) to head (0fdbf03).

Files	Patch %	Lines
...e/Unified/Internal/Class/UnifiedSimpleMergeable.hs	9.46%	151 Missing and 2 partials ⚠️
src/Grisette/Internal/Core/Data/Class/Mergeable.hs	68.09%	53 Missing and 14 partials ⚠️
...isette/Internal/Core/Data/Class/SimpleMergeable.hs	42.85%	12 Missing and 4 partials ⚠️
src/Grisette/Unified/Internal/Class/UnifiedSOrd.hs	0.00%	8 Missing ⚠️
...rc/Grisette/Unified/Internal/Class/UnifiedITEOp.hs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
- Coverage   53.85%   53.32%   -0.53%     
==========================================
  Files          97       96       -1     
  Lines        7675     7828     +153     
  Branches      658      664       +6     
==========================================
+ Hits         4133     4174      +41     
- Misses       2884     2990     +106     
- Partials      658      664       +6     

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[lsrcz]

This pull request also works towards #124.