[#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/ecdsa/init.go

@@ -0,0 +1,16 @@
+package neofsecdsa
+
+import neofscrypto "github.com/nspcc-dev/neofs-sdk-go/crypto"
+
+func init() {
+	neofscrypto.RegisterScheme(neofscrypto.ECDSA_SHA512, func() neofscrypto.PublicKey {
+		return new(PublicKey)
+	})
+
+	neofscrypto.RegisterScheme(neofscrypto.ECDSA_DETERMINISTIC_SHA256, func() neofscrypto.PublicKey {
+		var key PublicKey
+		key.MakeDeterministic()
+
+		return &key
+	})
+}