func makeBigInt(x *big.Int) Int {
	return Int{intImpl{big_: x}}
}
Precondition: x cannot be represented as int32.
package starlark
import "math/big"
type intImpl struct {
	small_ int64     
	big_   *big.Int  
}
func (i Int) get() (small int64, big *big.Int) {
	return i.impl.small_, i.impl.big_
}
func makeSmallInt(x int64) Int {
	return Int{intImpl{small_: x}}
}