chore(library/hott) move function extensionality into new axioms folder, adjust file(s) using it
feat(library/hott) add univalence axiom
chore(library/hott) adapted univalence axiom to suit notation in book and def in Coq.
Added substitution lemma for equivalence calculations
chore(library/hott) add note referring to missing substitution lemma
chore(library/hott) cleanup
fix(library/hott) make universe index in ua.lean variable