support `open_in` with an URL argument

[gasche]

The (very real) use case is a desperate university student with no computer at home, and only access to a public computer in his or her school's library, severely restricted (basic Caml installation, no right to install additional libraries or basically write any file on the computer), willing to work on image manipulation in OCaml.

The following could work in TryOCaml:

let image = open_in_bin "http://orion.math.iastate.edu/burkardt/data/bmp/snail.bmp";;
(* then use parsing code from http://pauillac.inria.fr/~cheno/lettre/lettre1.html *)

Current (incorrect) output:

Exception:
Sys_error
 "http://orion.math.iastate.edu/burkardt/data/bmp/snail.bmp: no such file or directory".

[hhugo]

Sys_js.register_autoload allows to do that (with CORS restriction https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS) see https://github.com/ocsigen/js_of_ocaml/blob/master/toplevel/toplevel.ml#L146

In the js_of_ocaml toplevel, the following will load http://ocsigen.org/resources/logos/text_js_of_ocaml_with_shadow.png let image = open_in_bin "/http/ocsigen.org/resources/logos/text_js_of_ocaml_with_shadow.png"

[cpitclaudel]

@hhugo Is this documented somewhere? Thanks!