Encapsulate EquivalenceClass into a struct (#8034)

* Minor: Encapsulate EquivalenceClass

* Rename inner to exprs

* Rename new_from_vec to new

* Apply suggestions from code review

Co-authored-by: Mehmet Ozan Kabak <ozankabak@gmail.com>

* treat as set rather than vec

* fmt

* clippy

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Co-authored-by: Mehmet Ozan Kabak <ozankabak@gmail.com>

Author: alamb