Big integers are unmarshalled to float with precision of 16

[andrehaland]

When trying to unmarshal a big integer the value is converted to a float with precision of 16.

Reproduce

https://play.golang.org/p/EJSeNwrMxVK

package main

import (
	"fmt"
	"gopkg.in/yaml.v3"
)

func main() {

	y := []byte("x: 123456789123456789123")
	var yamlObj interface{}
	err := yaml.Unmarshal(y, &yamlObj)
	if err != nil {
		panic(err)
	}
	fmt.Println("Before unmarshal: ", string(y))
	fmt.Println("After unmarshal: ", yamlObj)
}

Actual

Before unmarshal:  x: 123456789123456789123
After unmarshal:  map[x:1.234567891234568e+20]

Expected

Before unmarshal:  x: 123456789123456789123
After unmarshal:  map[x:12345678912345689123]

[taliastocks]

It's unfortunate that this and #435 are still unresolved. My guess is this is complicated because the library doesn't want to deal with integer wraparound, but it even selects the tag incorrectly when unmarshalling into a yaml.Node. I wish there were a way to change the library behavior to select the tag according to the Yaml spec so that I could avoid checking if !!float nodes are parsable as integers myself.