code: public static Mapping<Long> longv(Constraint... constraints) {
          return new FieldMapping(
                  InputMode.SINGLE,
                  mkSimpleConverter(s ->
                      isEmptyStr(s) ? 0l : Long.parseLong(s)
                  ), new MappingMeta(MAPPING_LONG, Long.class)
              ).constraint(checking(Long::parseLong, "error.long", true))
                  .constraint(constraints);
          }
docs: (convert to Long) mapping
  @param constraints constraints
  @return new created mapping