feat(category_theory/subobject): API for working with inequalities (#6818)
feat(category_theory/subobject): API for working with inequalities (#6818)

This PR adds two types of declarations:
* Helper functions for showing that two subobjects are equal by giving a compatible isomorphism, and
* functions `of_le`/`of_le_mk`/`of_mk_le`/`of_mk_le_mk` that produce a morphism between the underlying objects from a proof of `X ≤ Y`. These are in essence just thin wrappers around `underlying.map`.





leanprover-community/mathlib,leanprover-community/mathlib