Moosex::oneargnew lets your constructor take a single argument, which will be translated into the value for a one-entry hashref. it is a parameterized role with two parameters:
* type - the moose type that the single argument must be for the one-arg form
to work. this should be an existing type, and may be either a string type or
a moosex::type.
* init_arg - this is the string that will be used as the key for the hashref
constructed from the one-arg call to new.