Replace *KoblitzCurve by elliptic.Curve

Found via github.com/mvdan/interfacer.

btcec/privkey.go

@@ -6,6 +6,7 @@ package btcec
 
 import (
 	"crypto/ecdsa"
+	"crypto/elliptic"
 	"crypto/rand"
 	"math/big"
 )
@@ -17,7 +18,7 @@ type PrivateKey ecdsa.PrivateKey
 
 // PrivKeyFromBytes returns a private and public key for `curve' based on the
 // private key passed as an argument as a byte slice.
-func PrivKeyFromBytes(curve *KoblitzCurve, pk []byte) (*PrivateKey,
+func PrivKeyFromBytes(curve elliptic.Curve, pk []byte) (*PrivateKey,
 	*PublicKey) {
 	x, y := curve.ScalarBaseMult(pk)
 
@@ -35,7 +36,7 @@ func PrivKeyFromBytes(curve *KoblitzCurve, pk []byte) (*PrivateKey,
 
 // NewPrivateKey is a wrapper for ecdsa.GenerateKey that returns a PrivateKey
 // instead of the normal ecdsa.PrivateKey.
-func NewPrivateKey(curve *KoblitzCurve) (*PrivateKey, error) {
+func NewPrivateKey(curve elliptic.Curve) (*PrivateKey, error) {
 	key, err := ecdsa.GenerateKey(curve, rand.Reader)
 	if err != nil {
 		return nil, err