code: public static Mapping<BigInteger> bigInt(Constraint... constraints) {
          return new FieldMapping(
                  InputMode.SINGLE,
                  mkSimpleConverter(s ->
                      isEmptyStr(s) ? BigInteger.ZERO : new BigInteger(s)
                  ), new MappingMeta(MAPPING_BIG_INTEGER, BigInteger.class)
              ).constraint(checking(BigInteger::new, "error.bigint", true))
                  .constraint(constraints);
          }
docs: (convert to BigInteger) mapping
  @param constraints constraints
  @return new created mapping