refactor(library): rename exists_elim and exists_intro to exists.elim
and exists.intro
refactor(library): rename theorems "iff.flip_sign -> not_iff_not_of_iff" and "decidable_iff_equiv -> decidable_of_decidable_of_iff"
refactor(library): rename theorems "iff.flip_sign -> not_iff_not_of_iff" and "decidable_iff_equiv -> decidable_of_decidable_of_iff"