code: public static NestableFieldValidator mapFv(Class key, Class val, boolean nullAllowed) {
          return mapFv(fv(key, false), fv(val, false), nullAllowed);
      }
docs: Returns a new NestableFieldValidator for a Map of key to val.
  
  @param key         the Class of keys in the map
  @param val         the Class of values in the map
  @param nullAllowed whether or not a value of null is valid
  @return a NestableFieldValidator for a Map of key to val