Unboxed tuples fixes

[Ekdohibs]

Unboxed tuples appearing in free variables of closures were previously failing. This PR can be tested with Ekdohibs@dcd9995 which sits on top of the unboxed tuples frontend work.
Example code that was previously failing:

type t = #(int * int)

let id_t (x : t) = x

let app f (x : t) = f x

let f y = app (fun _ -> id_t y) y

[ccasin]

I've tested this PR on the unboxed tuples branch (see test 3 here, which passes). It will need an flambda2 reviewer, but waiting for @mshinwell's return is OK if no one is available.

[Ekdohibs]

Note: I added fixes to other problems with unboxed products in lambda_to_flambda.