func SetBackend(b Backend) {
	if backend != nil {
		panic("wallet backend already set")
	}
	backend = b
}
SetBackend sets the global wallet backend. Must not be called directly but
through importing the needed backend.
package wallet
import (
	"io"
)
var backend Backend
type Backend interface {
	DecodeAddress(io.Reader) (Address, error)
	DecodeSig(io.Reader) (Sig, error)
	VerifySignature(msg []byte, sign Sig, a Address) (bool, error)
}