Add a primitive `Let` that is compiled to `let` in Haskell

[jespercockx]

[flupe]

May I suggest having a cursed syntax:

infixr 5 Let
syntax Let a (λ x → b) = let[ x ← a ] b

t : Bool
t =
  let[ x ← True  ]
  let[ y ← False ]
  x || y

[flupe]

Note: Haskell's let bindings are always recursive, so we should make sure that the name used in the Let does not appear in the expression being bound to prevent incorrect behavior.