func checksum(input []byte) (cksum [4]byte) {
	h := sha256.Sum256(input)
	h2 := sha256.Sum256(h[:])
	copy(cksum[:], h2[:4])
	return
}
checksum: first four bytes of sha256^2
package base58
import (
	"crypto/sha256"
	"errors"
)
var ErrChecksum = errors.New("checksum error")
var ErrInvalidFormat = errors.New("invalid format: version and/or checksum bytes missing")