Ok, really nothing to see here, some Pipe abstraction on top of Lwt_io channels

Why did I do this thing, and not using directly Lwt_io channels (or even better, Async Pipes)?

Because of things like this ocsigen/lwt#345
Quite easy to do it wrong, so for very basic operations when working with local data,
we can use this (I surely am), brings a little bit more of a type safety
into all that write/read value to channels.

How to build:

opam install dune lwt
make

How to use:

>  module Pipe = Tube.Make(struct type t = string end);;
module Pipe :
	sig
		type t = string
		type reader
		type writer
		val create : unit -> reader * writer
		val write : t -> writer -> unit Lwt.t
		val read : reader -> t Lwt.t
	end
> let (reader, writer) = Pipe.create ();;
val reader : Pipe.reader = <abstr>
val writer : Pipe.writer = <abstr>
> Pipe.write "something" writer;;
- : unit = ()
> Lwt.(Pipe.read reader >>= fun s -> Lwt_io.printlf "My fancy value: %s" s);;
My fancy value: something
- : unit = ()
> Pipe.write 911 writer;;
Error: This expression has type int but an expression was expected of type
				string

or, for write with pushback:

> let (reader, writer) = Pipe.create ();;
val reader : Pipe.reader = <abstr>
val writer : Pipe.writer = <abstr>
> Pipe.write_with_pushback "something" writer;;
(* This will block, when using inside of `utop`, till something reads from the created `reader`; *)
(* when doing this write in a different context than `utop`, the produced Lwt.t will not move on *)
(* until something will read from the `reader` *)

or, for custom data types:

> type roman_numeral = I | II | III | IV | V | VI | VII | VIII | IX | X;;
type roman_numeral = I | II | III | IV | V | VI | VII | VIII | IX | X
> module Pipe = Tube.Make(struct type t = roman_numeral end);;
module Pipe :
	sig
		type t = roman_numeral
		type reader
		type writer
		val create : unit -> reader * writer
		val write : t -> writer -> unit Lwt.t
		val read : reader -> t Lwt.t
	end
> let (reader, writer) = Pipe.create ();;
val reader : Pipe.reader = <abstr>
val writer : Pipe.writer = <abstr>
> Pipe.write IX writer;;
- : unit = ()
> Lwt.(Pipe.read reader >>= fun s ->
match s with
| IX -> Lwt_io.printl "9"
| _ -> Lwt_io.printl "Don't care...");;
9
- : unit = ()
> Pipe.write "not cool" writer;;
Error: This expression has type string but an expression was expected of type
				roman_numeral