feat(algebraic_topology/simplex_category): epi and monos in the simplex category (#8101)
feat(algebraic_topology/simplex_category): epi and monos in the simplex category (#8101)

Characterize epimorphisms and monomorphisms in `simplex_category` in terms of the function they represent. Add lemmas about their behavior on length of objects. 



Co-authored-by: Robin Carlier <40955929ce374016dc6c7df3325369bbe58400ea@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib