Get rid of = private Cvar.t

[imeckler]

definitions like

module Boolean : sig
  type var = private Cvar.t
end

are ultimately I think bad, because the coercion to Cvar.t leaves implicit the mapping between booleans and field elements. It would be better to have a function Boolean.to_zero_one : Boolean.var -> Cvar.t

[imeckler]

My opinion on this has changed. I now think of a Boolean not as a bool but as a field element which is either 0 or 1.