Provide Sigma compiled to tuples

[Lucas Escot]

record Sigma (a : Set) (b : @0 a → Set) : Set where
  constructor Pair
  field
    fst : a
    snd : b fst
{-# COMPILE AGDA2HS Sigma #-}

(The builtin dependent sigma type, properly annotated).

[Jesper Cockx]

Looks good. Once we have #105 we could make it compile to actual Haskell pairs.

[Jesper Cockx]

See https://github.com/jespercockx/agda-core/blob/main/src/Agda/Core/Utils.agda#L14-L29