Add predecessor for natural numbers

[jespercockx]

For reasons (TM) I wanted to implement a recursor for natural numbers in agda2hs. Since pattern matching on Natural is not allowed in Haskell, I thought I'd use ifDec instead. However, in the non-zero branch I need to take the predecessor in order to make the recursive call:

recNat : (a : @0 Nat → Set)
       → (z : a 0)
       → (s : (m : Nat) → a m → a (suc m))
       → (n : Nat) → a n
recNat a z s n = ifDec (n ≟ 0)
  (λ where {{refl}} → z)
  (λ {{n≠0}} →
    case predNat n n≠0 of λ where
      (m ⟨ refl ⟩) → s m (recNat a z s m))

{-# COMPILE AGDA2HS recNat #-}

predNat needs a somewhat complicated (but sensible) type:

predNat : (n : Nat) → @0 (n ≡ 0 → ⊥) → ∃ Nat λ m → n ≡ suc m

Since it doesn't seem possible to implement predNat with the primitives that agda2hs currently provides, I thought it could be nice to add it as a primitive, so more complex functions like recNat can be implemented on top. WDYT?

(The definition of _≟_ is currently missing from the agda2hs prelude, but it can readily be defined:

_≟_ : {{_ : Eq a}} {{_ : IsLawfulEq a}} → (x y : a) → Dec (x ≡ y)
x ≟ y = (x == y) ⟨ isEquality x y ⟩

{-# COMPILE AGDA2HS _≟_ inline #-}

)