Lin: support tuple combinators

[jmid]

As listed in Lin's current limitations in the manual page https://ocaml-multicore.github.io/multicoretests/0.1/qcheck-lin/#current-limitations it currently doesn't have combinators for tuples akin to the usual * type syntax.

This hasn't been much of a problem so far, since OCaml Stdlib functions are generally curried.
For the first time, I hit this limitation in practice for a result type in Weak as part of #214 which contains following function signature:

Weak.S.stats : t -> int * int * int * int * int * int

We should experiment with tuple combinator syntax to see how close we can come to the above 🤔