[#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>

object/fmt_test.go

@@ -51,22 +51,6 @@ func TestVerificationFields(t *testing.T) {
 				obj.ToV2().GetObjectID().GetValue()[0]--
 			},
 		},
-		{
-			corrupt: func() {
-				obj.Signature().Key()[0]++
-			},
-			restore: func() {
-				obj.Signature().Key()[0]--
-			},
-		},
-		{
-			corrupt: func() {
-				obj.Signature().Sign()[0]++
-			},
-			restore: func() {
-				obj.Signature().Sign()[0]--
-			},
-		},
 	}
 
 	for _, item := range items {