Default modalities and simple modal modules

[tdelvecchio-jsc]

Enables adding @@ portable at the top of an mli file (and at the beginning of sig ... end blocks), as well as marking module declarations with modes:

@@ portable

val portable : unit -> unit
val nonportable : unit -> unit @@ nonportable

module (T @ nonportable) : sig @@ portable
  val portable : unit -> unit
  val nonportable : unit -> unit @@ nonportable
end

This is enough to get us started adding portability to our code, though there is more syntax to implement in the future.