Support @@cfg within module & signatures

[leostera]

Right now we can't use @cfg in elements defined within modules and signatures.

The expected behavior is that @cfg can be used in places like this:

module A : sig
 val a : unit -> unit [@@cfg disable = true]
end =
 let a () = () [@@cfg disable = true]
end

and interface files:

module A : sig
 val a : unit -> unit [@@cfg disable = true]
end

[KFoxder]

@leostera you can assign this to me. I am going to put something out for it.

[KFoxder]

@leostera this can be closed now I think