Fix Set::equals() loading its data when not necessary

[Baptouuuu]

Problem

Closes #56

Unlike mentioned in the issue this is not problematic for formal/orm as it compares the objects identity before calling the equals method.

But this problem could still exist elsewhere.

Solution

Add a check at the start of the method to make an early return if the other Set is itself.

Note

The same problem applied to Sequence::equals() and has been fixed the same way.

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 4 lines in your changes missing coverage. Please review.

Project coverage is 97.07%. Comparing base (2db4729) to head (5b0f197).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
proofs/sequence.php	84.61%	2 Missing ⚠️
proofs/set.php	84.61%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop      #57      +/-   ##
=============================================
- Coverage      97.24%   97.07%   -0.18%     
- Complexity      1062     1064       +2     
=============================================
  Files             75       75              
  Lines           5268     5298      +30     
=============================================
+ Hits            5123     5143      +20     
- Misses           145      155      +10     

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