GROOVY-11775: ExpandoMetaClass: prevent mixin metaclass duplication

[eric-milles]

MixinInMetaClass used super.equals() (in Groovy 5 and prior) and super.hashCode() which resulted in identity-only differentiation. Implement equals and hashCode in terms of mixinClass and emc. I chose to keep the hash code simple; MMC instances will bucket up according to their mixin class.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 68.18182% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.9900%. Comparing base (b6df8a1) to head (961370c).

Files with missing lines	Patch %	Lines
...g/codehaus/groovy/reflection/MixinInMetaClass.java	68.4210%	4 Missing and 2 partials ⚠️
src/main/java/groovy/lang/ExpandoMetaClass.java	66.6667%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2335        +/-   ##
==================================================
+ Coverage     66.9894%   66.9900%   +0.0006%     
- Complexity      29330      29331         +1     
==================================================
  Files            1382       1382                
  Lines          116614     116607         -7     
  Branches        20432      20429         -3     
==================================================
- Hits            78119      78115         -4     
+ Misses          32061      32059         -2     
+ Partials         6434       6433         -1     

Files with missing lines	Coverage Δ
src/main/java/groovy/lang/ExpandoMetaClass.java	76.6187% <66.6667%> (-0.0420%)	⬇️
...g/codehaus/groovy/reflection/MixinInMetaClass.java	78.6667% <68.4210%> (+2.1235%)	⬆️

... and 2 files with indirect coverage changes

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