Pull up ValidatorMethod.

    * Will be referenced within the class definition(generation) step.
Fix Froshki class for base-classing usages.

    * Add `Prebuilt` metaclass, which instantiates the target class
      automatically right after the definition time.
    * Apply this metaclass to Froshki.  This guarantees Froshki.__new__
      to be called before any subclassing without manual instantiations.
Increment version to 0.4.1.