Empty commit after rebasing (???
Functoriality of [forall].
Added [flip] and [isequiv_flip].
Reinstated [equiv_rect], [equiv_intro], accidentally deleted when rebasing.
Added [Trunc_forall].
Added [Trunc_resp_equiv], modulo outstanding lemma [ap_equiv].
Added [ap_equiv]: an equivalence induces an equivalence on paths.