add big.Int Set, Abs, Neg and add test it

_cmptest/bigintdemo/fib.go

@@ -5,7 +5,7 @@ import (
 	"math/big"
 )
 
-func main() {
+func fib() {
 	// Initialize two big ints with the first two numbers in the sequence.
 	a := big.NewInt(0)
 	b := big.NewInt(1)
@@ -23,3 +23,30 @@ func main() {
 	}
 	fmt.Println(a) // 100-digit Fibonacci number
 }
+
+func abs() {
+	a := big.NewInt(64)
+	b := big.NewInt(-52)
+	a.Set(b)
+	a.Abs(a)
+	a.Set(big.NewInt(-164))
+	a.Abs(a)
+	fmt.Println("value: ", a.String())
+}
+
+func neg() {
+	fmt.Println("value: ", big.NewInt(-64).Neg(big.NewInt(-64)))
+	fmt.Println("value: ", big.NewInt(64).Neg(big.NewInt(64)))
+	fmt.Println("value: ", big.NewInt(0).Neg(big.NewInt(0)))
+}
+
+func main() {
+	a := big.NewInt(64)
+	b := big.NewInt(-52)
+	c := big.NewInt(54)
+	fmt.Println("value:", a.Add(a, b))
+	fmt.Println("value:", a.Sub(b, c))
+	d := big.NewInt(10)
+	e := big.NewInt(4)
+	fmt.Println("value:", d.Mul(d, e))
+}