Calling a message `Unit` yields uncompileable code

[Leonidas-from-XIV]

It gets translated into unit which then fails to compile because the .ml and the .mli file disagree on what unit refers to and yields confusing errors like:

Error: The implementation src/licensing/licensing_types.pp.ml
       does not match the interface src/licensing/.licensing.objs/licensing__Licensing_types.cmi:
       Values do not match:
         val default_unit_unlimited : unit -> unit_unlimited
       is not included in
         val default_unit_unlimited : unit -> unit_unlimited
       File "src/licensing/licensing_types.mli", line 73, characters 0-51:
         Expected declaration
       File "src/licensing/licensing_types.ml", line 67, characters 8-30:
         Actual declaration

[mransan]

Fixed in #145 will start an opam release.