Add support for Lwt_preemptive

[yallop]

This adds an extra value, Cstubs.lwt_preemptive, to the interface:

val lwt_preemptive : concurrency_policy
(** Generate code which runs C function calls with the Lwt_preemptive module:

    http://ocsigen.org/lwt/2.5.1/api/Lwt_preemptive

    Passing [lwt_preemptive] as the [concurrency] argument to {!Cstubs.write_c} and
    {!Cstubs.write_ml} changes the return type of bound functions to include
    the {!Lwt.t} constructor.  For example, the binding specification

       [let unlink = foreign "unlink" (string @-> returning int)]

    generates a value of the following type by default:

       [val unlink : string -> int]

    but when using [lwt_preemptive] the generated type is as follows:

       [val unlink : string -> int Lwt.t]

    Additionally, the OCaml runtime lock is released during calls to functions
    bound with [lwt_preemptive].
*)

Passing ~concurrency_policy:Cstubs.lwt_preemptive to the write_c and write_ml functions results in generated code that runs bound C functions with Lwt_preemptive.detach.