Add Haskell.Data.List

[HeinrichApfelmus]

The purpose of this pull request is to add some functions from the module Data.List. Specifically, we add

• nub with a proof of a property prop-elem-nub
• deleteAll as a helper function for nub
• sort as a postulate

We also add Haskell.Data.Ord, this is needed for sortOn.

I quite like the property

prop-elem-nub
  : ∀ ⦃ _ : Eq a ⦄ ⦃ _ : IsLawfulEq a ⦄
      (x : a) (ys : List a)
  → elem x (nub ys)
    ≡ elem x ys

as it expresses a logical equivalence in terms of ≡, I think that this is only available for Bool, not for Prop / Set. The proof is also very succinct.

On the other hand, the proof for prop-elem-deleteAll is somewhat lengthy, but I'm not sure that this can be avoided, as we have to use transitivity of == at crucial points in the proof. I am using the induction hypothesis prop-elem-deleteAll x y zs in a with clause in order to avoid doing substitutions by hand via cong.

[jespercockx]

Thanks for the PR! I have a few minor nitpicks, but otherwise this looks very good.

[omelkonian]

LGTM