stackitem: introduce Convertible interface

We have a lot of native contract types that are converted to stack items
before serialization, then deserialized as stack items and converted back to
regular structures. stackitem.Convertible allows to remove a lot of repetitive
io.Serializable code.

This also introduces to/from converter in testserdes which unfortunately
required to change util tests to avoid circular references.

internal/testserdes/testing.go

@@ -5,6 +5,7 @@ import (
 	"testing"
 
 	"github.com/nspcc-dev/neo-go/pkg/io"
+	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
 	"github.com/stretchr/testify/require"
 )
 
@@ -26,6 +27,15 @@ func EncodeDecodeBinary(t *testing.T, expected, actual io.Serializable) {
 	require.Equal(t, expected, actual)
 }
 
+// ToFromStackItem checks if expected stays the same after converting to/from
+// StackItem.
+func ToFromStackItem(t *testing.T, expected, actual stackitem.Convertible) {
+	item, err := expected.ToStackItem()
+	require.NoError(t, err)
+	require.NoError(t, actual.FromStackItem(item))
+	require.Equal(t, expected, actual)
+}
+
 // EncodeBinary serializes a to a byte slice.
 func EncodeBinary(a io.Serializable) ([]byte, error) {
 	w := io.NewBufBinWriter()