feat(library/data/sum): use + notation for sum
feat(library/algebra): add to developments of group, ordered_group, and ring
feat(library): reorganize and declare some notation
feat(library/binary, library/relation): introduce general properties for binary operations and relations
refactor(library/algebra/relation, library/logic/instances): revise equivalence relations and congruences to use structure command
fix(library/algebra/category/morphism): remove sorry that was introduced by accident
fix(library/data/prod/decl): give preference to unicode version when pretty printing
fix(tests/lean): to reflect changes in the standard library