mathlib3

89924726 - fix(category_theory): make the `nat_trans` arrow `⟹` a synonym for the `hom` arrow (#907)

fix(category_theory): make the `nat_trans` arrow `⟹` a synonym for the `hom` arrow (#907) * removing the nat_trans and vcomp notations; use \hom and \gg * a simpler proposal * getting rid of vcomp * fix * update notations in documentation * typo in docs