public static BigDecimal fastBigDecimalValue(
      int fastSignum, long fast0, long fast1, long fast2,
      int fastIntegerDigitCount, int fastScale) {
    BigInteger unscaledValue =
        fastBigIntegerValueUnscaled(
            fastSignum, fast0, fast1, fast2);
    return new BigDecimal(unscaledValue, fastScale);
  }
Return a BigDecimal representing the decimal.  The BigDecimal class is able to accurately
represent the decimal.

NOTE: We are not representing our decimal as BigDecimal now as OldHiveDecimal did, so this
is now slower.

@param fastSignum the sign (-1, 0, or +1)
@param fast0 word 0 of the internal representation
@param fast1 word 1
@param fast2 word 2
@param fastIntegerDigitCount the number of integer digits
@param fastScale the scale of the number
@return the BigDecimal equivalent
storage-api/src/java/org/apache/hadoop/hive/common/type/FastHiveDecimalImpl.java