mathlib3

0ccd2f66 - feat(data/dfinsupp): add simp lemma `single_eq_zero` (#8447)

feat(data/dfinsupp): add simp lemma `single_eq_zero` (#8447) This matches `finsupp.single_eq_zero`. Also adds `dfinsupp.ext_iff`, and changes some lemma arguments to be explicit.