crypto: drop home-grown elliptic crypto, use crypto/elliptic

As NEO uses P256 we can use standard crypto/elliptic library for almost
everything, the only exception being decompression of the Y coordinate. For
some reason the standard library only supports uncompressed format in its
Marshal()/Unmarshal() functions. elliptic.P256() is known to have
constant-time implementation, so it fixes #245 (and the decompression using
big.Int operates on public key, so nobody really cares about that part being
constant-time).

New decompress function is inspired by
https://stackoverflow.com/questions/46283760, even though the previous one
really did the same thing just in a little less obvious way.

pkg/crypto/keys/publickey_test.go

@@ -5,12 +5,11 @@ import (
 	"encoding/hex"
 	"testing"
 
-	"github.com/CityOfZion/neo-go/pkg/crypto"
 	"github.com/stretchr/testify/assert"
 )
 
 func TestEncodeDecodeInfinity(t *testing.T) {
-	key := &PublicKey{crypto.ECPoint{}}
+	key := &PublicKey{}
 	buf := new(bytes.Buffer)
 	assert.Nil(t, key.EncodeBinary(buf))
 	assert.Equal(t, 1, buf.Len())