[FIX] Fix variables equality and hashes

[PrimozGodec]

Issue

Fixes (partially or completely) #4870

Problem is the following:

from Orange.data import Domain, DiscreteVariable
a = DiscreteVariable("Cluster", values=["c"])
var1 = a.renamed("Cluster x")
var2 = DiscreteVariable("Cluster", values=["a", "b"])
domain = Domain([], metas=[var1, var2])

print(var1 == var2)  # vars have completely different origin but still equal
>>> True
print(domain[var1]) # wrong attribute because of wrong comparison -> should return Cluster x
>>> Cluster

The problem is that var1 and var2 have equal hash (also __eq__ returns True). This happens since var1 have compute_value with identity pointing to the variable a; variables a and var2 have same hash since they have the same name and same compute_value (None).

The Domain has a dictionary with index for each variable. Variables are keys in dict, when updating dictionary index is set for variable Cluster x instead of Cluster (same hashes).

Description of changes

Variables are not equal if their names are not equal and their hashes are not equal either. This avoids problems in the domain (two vars with same hash).

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4957 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4957   +/-   ##
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  Coverage   84.28%   84.28%           
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  Files         283      283           
  Lines       58016    58016           
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+ Hits        48898    48900    +2     
+ Misses       9118     9116    -2