feat(init/funext): add function extensionality as an axiom.
feat(init/funext): add function extensionality as an axiom.

This allows us to keep track of theorems which use function extensionality, but not full univalence

javra/lean,UlrikBuchholtz/lean,fpvandoorn/lean2,levnach/lean,fpvandoorn/lean2,fpvandoorn/lean2,javra/lean,soonhokong/lean,soonhokong/lean,UlrikBuchholtz/lean,fpvandoorn/lean2,UlrikBuchholtz/lean,levnach/lean,UlrikBuchholtz/lean,javra/lean,fpvandoorn/lean2,javra/lean,levnach/lean,soonhokong/lean,javra/lean,levnach/lean,levnach/lean,UlrikBuchholtz/lean,soonhokong/lean,soonhokong/lean