Fix Definition of Instance/Poset.v

[t-wissmann]

Fix a couple of problems in the definition of Poset Category, amounting to the following changes:

• C should be a Type of the objects (not: a Category)
• A Poset is Antisymmetric (not: Asymmetric)
• Since Antisymmetric in Coq is parametric in an equivalence relation, we just fix this equivalence to be propositional equality.
• Add (ℕ,≤) as an example

[t-wissmann]

The new type of @Poset is:

@Poset
     : ∀ (C : Type) (R : relation C), PreOrder R → Antisymmetric C eq R → Category

[t-wissmann]

With this PR I'd like to fix the issues in the current definition. Still, since antisymmetry isn't used at all in the definition of Poset, I wonder what the benefit of the definition is, anway.

[t-wissmann]

Thanks!