Support for simple module construction.

[sbriais]

This PR adds support for

• module aliases module A = B

This can be convenient when writing interfaces and referring to other components.

For instance, in another mli

module Event = Js_browser.Event

val foo: Event.t -> string -> unit [@@js.call]

This way, we can just write Event and not Js_browser.Event everywhere.

[mlasson]

i added a small piece of documentation, feel to free to merge or amend it !