Visible type/kind applications in declaration of data/type constructors

I'm making this ticket to keep track of this, I don't know if it's a good idea.

Allow visible type/kind applications when declaring data/type constructors on the LHS of :: (from Phab comment).

data
    Proxy @k :: k -> Type where
  MkProxy @k :: Proxy @k (a :: k)

&

data
  Fin     :: N -> Type where
  FinO @n ::          Fin (S n)
  FinS @n :: Fin n -> Fin (S n)

&

data
  Elem  @k @n  :: Vec n k -> Type where
  ElemO @a @as :: Elem @k @(S n) (a:>as)
  ElemS @a @as :: Elem @k @n     as
               -> Elem @k @(S n) (a:>as)