hdkeychain: Use secp256k1 privkey to pubkey method.

This makes use of the PubKey method on secp256k1.PrivateKey to convert a
private key to a public key instead of manually performing the
conversion.

This is desirable since secp256k1 provides the method specifically for
this purpose and it also has the benefit of allowing secp256k1 to
perform the task without round tripping through big integers which
results in slightly faster derivation.

The following benchmarks show a before and after comparison of the
affected operations:

benchmark                 old ns/op    new ns/op    delta
----------------------------------------------------------
BenchmarkDeriveHardened   3702         3671         -0.84%
BenchmarkDeriveNormal     4053         3999         -1.33%

benchmark                 old allocs   new allocs   delta
----------------------------------------------------------
BenchmarkDeriveHardened   14           14           +0.00%
BenchmarkDeriveNormal     15           15           +0.00%

benchmark                 old bytes    new bytes    delta
----------------------------------------------------------
BenchmarkDeriveHardened   1360         1360         +0.00%
BenchmarkDeriveNormal     1409         1409         +0.00%

hdkeychain/extendedkey.go

@@ -164,9 +164,8 @@ func (k *ExtendedKey) pubKeyBytes() []byte {
 	// This is a private extended key, so calculate and memoize the public
 	// key if needed.
 	if len(k.pubKey) == 0 {
-		pkx, pky := secp256k1.S256().ScalarBaseMult(k.key)
-		pubKey := secp256k1.NewPublicKey(pkx, pky)
-		k.pubKey = pubKey.SerializeCompressed()
+		privKey := secp256k1.PrivKeyFromBytes(k.key)
+		k.pubKey = privKey.PubKey().SerializeCompressed()
 	}
 
 	return k.pubKey