[#190] Refactor cryptographic functionality

Remove `signature` and `util/signature` packages. Re-implement their
functionality in new `crypto` package. Generalize the approach of
digital signature computation and verification by adding `Signer` and
`PublicKey` primitives similar to standard `crypto` package. Support
already exising in protocol signature schemes.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

crypto/crypto_test.go

@@ -0,0 +1,51 @@
+package neofscrypto_test
+
+import (
+	"math/rand"
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
+	"github.com/nspcc-dev/neofs-api-go/v2/refs"
+	neofscrypto "github.com/nspcc-dev/neofs-sdk-go/crypto"
+	neofsecdsa "github.com/nspcc-dev/neofs-sdk-go/crypto/ecdsa"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSignature(t *testing.T) {
+	data := make([]byte, 512)
+	rand.Read(data)
+
+	k, err := keys.NewPrivateKey()
+	require.NoError(t, err)
+
+	var s neofscrypto.Signature
+	var m refs.Signature
+
+	for _, f := range []func() neofscrypto.Signer{
+		func() neofscrypto.Signer {
+			var key neofsecdsa.Signer
+			key.SetKey(k.PrivateKey)
+
+			return &key
+		},
+		func() neofscrypto.Signer {
+			var key neofsecdsa.Signer
+			key.SetKey(k.PrivateKey)
+			key.MakeDeterministic()
+
+			return &key
+		},
+	} {
+		signer := f()
+
+		err := s.Calculate(signer, data)
+		require.NoError(t, err)
+
+		s.WriteToV2(&m)
+
+		s.ReadFromV2(m)
+
+		valid := s.Verify(data)
+		require.True(t, valid)
+	}
+}