Add support for Iarray.t as a testable type

[JanBeh]

As Iarray.t is exposed as part of OCaml's standard library (version 5.4) now, it would be nice if a corresponding Alcotest.iarray was added to Alcotest:

val 'a testable -> 'a Iarray.t testable

Note that OCaml's documentation currently wrongly exposes iarray as a type, which seems to be an implementation detail though that shouldn't have been exposed (see this forum post). So Iarray.t should be used instead when referring to OCaml's immutable array type (of course, the testable could still be named Alcotest.iarray).