func (x Target) AddDifficulties(y Target) (t Target) {
	sumDifficulty := new(big.Rat).Add(x.Inverse(), y.Inverse())
	return RatToTarget(new(big.Rat).Inv(sumDifficulty))
}
AddDifficulties returns the resulting target with the difficulty of 'x' and
'y' are added together. Note that the difficulty is the inverse of the
target. The sum is defined by:
sum(x, y) = 1/(1/x + 1/y)
package types
import (
	"errors"
	"math/big"
	"github.com/NebulousLabs/Sia/build"
	"github.com/NebulousLabs/Sia/crypto"
)
type (
	Target crypto.Hash
)
var (
	ErrNegativeTarget = errors.New("negative value used when converting to target")
)