How to convert int to bigint in golang?

I'm trying to implement fast double Fibonacci algorithm as described here:

// Fast doubling Fibonacci algorithm
package main

import "fmt"

//  (Public) Returns F(n).
func fibonacci(n int) int {
	if n < 0 {
		panic("Negative arguments not implemented")
	}
	fst, _ := fib(n)
	return fst
}

// (Private) Returns the tuple (F(n), F(n+1)).
func fib(n int) (int, int) {
	if n == 0 {
		return 0, 1
	}
	a, b := fib(n / 2)
	c := a * (b*2 - a)
	d := a*a + b*b
	if n%2 == 0 {
		return c, d
	} else {
		return d, c + d
	}
}

func main() {
	fmt.Println(fibonacci(13))
	fmt.Println(fibonacci(14))
}

This works fine for small numbers; however, when the input number get larger, the program returns a wrong result. So I tried to use bigInt from math/big package:

// Fast doubling Fibonacci algorithm
package main

import (
	"fmt"
	"math/big"
)

//  (Public) Returns F(n).
func fibonacci(n int) big.Int {
	if n < 0 {
		panic("Negative arguments not implemented")
	}
	fst, _ := fib(n)
	return fst
}

// (Private) Returns the tuple (F(n), F(n+1)).
func fib(n int) (big.Int, big.Int) {
	if n == 0 {
		return big.Int(0), big.Int(1)
	}
	a, b := fib(n / 2)
	c := a * (b*2 - a)
	d := a*a + b*b
	if n%2 == 0 {
		return c, d
	} else {
		return d, c + d
	}
}

func main() {
	fmt.Println(fibonacci(123))
	fmt.Println(fibonacci(124))
}

However, go build complains that

cannot convert 0 (type int) to type big.Int

How to mitigate this problem?

Use big.NewInt() instead of big.Int(). big.Int() is just type casting.
You need to check out documentation of big package
You should mostly use methods with form func (z *T) Binary(x, y *T) *T // z = x op y
To multiply 2 arguments you need to provide result variable, after it call Mul method. So, for example, to get result of 2*2 you need to:
big.NewInt(0).Mul(big.NewInt(2), big.NewInt(2))

You can try working example on the <kbd>Go playground</kbd>

Also you can create extension functions like:

func Mul(x, y *big.Int) *big.Int {
    return big.NewInt(0).Mul(x, y)
}

To make code more readable:

// Fast doubling Fibonacci algorithm
package main

import (
	"fmt"
	"math/big"
)

//  (Public) Returns F(n).
func fibonacci(n int) *big.Int {
	if n < 0 {
		panic("Negative arguments not implemented")
	}
	fst, _ := fib(n)
	return fst
}

// (Private) Returns the tuple (F(n), F(n+1)).
func fib(n int) (*big.Int, *big.Int) {
	if n == 0 {
		return big.NewInt(0), big.NewInt(1)
	}
	a, b := fib(n / 2)
	c := Mul(a, Sub(Mul(b, big.NewInt(2)), a))
	d := Add(Mul(a, a), Mul(b, b))
	if n%2 == 0 {
		return c, d
	} else {
		return d, Add(c, d)
	}
}

func main() {
	fmt.Println(fibonacci(123))
	fmt.Println(fibonacci(124))
}

func Mul(x, y *big.Int) *big.Int {
	return big.NewInt(0).Mul(x, y)
}
func Sub(x, y *big.Int) *big.Int {
	return big.NewInt(0).Sub(x, y)
}
func Add(x, y *big.Int) *big.Int {
	return big.NewInt(0).Add(x, y)
}

Try it on the <kbd>Go playground</kbd>