type BigInt big.Int

// String casts BigInt into big.Int and uses its String method.
func (b *BigInt) String() string {
    bc := big.Int(*b)
    return bc.String()
}

// UnmarshalJSON casts BigInt into big.Int and uses its UnmarshalJSON method.
func (b *BigInt) UnmarshalJSON(text []byte) error {
    bc := new(big.Int)
    err := bc.UnmarshalJSON(text)
    if err != nil {
        return err
    }
    b = b.Set(bc)
    return nil
}

// Set is different from big.Int.Set() in that you must use the value it returns
func (b *BigInt) Set(i *big.Int) *BigInt {
    iB := BigInt(*i)
    b = &iB
    return b
}