The simplest code is the code you didn't write.


package main

import (
    "bufio"
    "fmt"
    "math/big"
    "os"
)

var (
    bufout = bufio.NewWriter(os.Stdout)
    bufin = bufio.NewReader(os.Stdin)
)

func println(a ...interface{}) { fmt.Fprintln(bufout, a...) }
func scanf(f string, a ...interface{}) { fmt.Fscanf(bufin, f, a...) }

func main() {
    defer bufout.Flush()

    var a, b string
    scanf("%s %s", &a, &b)

    var x = new(big.Int)
    var y = new(big.Int)
    var z = new(big.Int)

    x.SetString(a, 10)
    y.SetString(b, 10)
    z.Mul(x, y)

    println(z)
}