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Eliminate and regroup duplicate definitions in Adjunctions and Equivalences #1983

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@arnoudvanderleer

Currently, both Equivalences.CompositesAndInverses and Equivalences.FullyFaithful each have a lemma about fully faithfulness and one about essentially surjectiveness, which should be grouped together and one should be derived from the other:

FullyFaithful.fully_faithful_from_equivalence
     : ∏ (A B : category) (F : A ⟶ B),
       adj_equivalence_of_cats F → fully_faithful F
CompositesAndInverses.right_adj_equiv_is_ff
     : ∏ (D1 D2 : category) (F : D1 ⟶ D2) (GG : adj_equivalence_of_cats F),
       fully_faithful (Core.right_adjoint GG)

FullyFaithful.functor_from_equivalence_is_essentially_surjective
     : ∏ (A B : category) (F : A ⟶ B),
       adj_equivalence_of_cats F → essentially_surjective F
CompositesAndInverses.right_adj_equiv_is_ess_sur
     : ∏ (D1 D2 : category) (F : D1 ⟶ D2) (GG : adj_equivalence_of_cats F),
       essentially_surjective (Core.right_adjoint GG)

Also, we have adj_equivalence_of_cats_inv both in Adjunctions.Core and Adjunctions.CompositesAndInverses. As long as we have a file CompositesAndInverses, it makes most sense to have the lemma there, but the one in Core is a one-line definition because Core already provides a lot of machinery for constructing equivalences in multiple different ways.

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