Allow destruction in let bindings' pattern

[panglesd]

Before, it was impossible to obliterate a pattern inside a let-binding (unless the let-binding was turned into a match internally of course)

type t = { a : int; b : int}
let v a b = { a; b }

(* Calling destruct on the _ below would fail *)
let _ : t = v 1 2

As far as I understand, there is no reason for this limitation, it was just not implemented. This PR fixes that.

[panglesd]

Related to: #1162

[voodoos]

Thanks @panglesd, that's cool ! The changes are surprisingly contained, and feel reasonable.