stackitem: rework error handling

Return good named errors everywhere, export appropriate constants, make
errors.Is() work.

pkg/vm/stackitem/serialization_test.go

@@ -1,6 +1,7 @@
 package stackitem
 
 import (
+	"errors"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -10,12 +11,13 @@ func TestSerializationMaxErr(t *testing.T) {
 	base := make([]byte, MaxSize/2+1)
 	item := Make(base)
 
-	arr := []Item{item, item.Dup()}
+	// Pointer is unserializable, but we specifically want to catch ErrTooBig.
+	arr := []Item{item, item.Dup(), NewPointer(0, []byte{})}
 	aitem := Make(arr)
 
 	_, err := Serialize(item)
 	require.NoError(t, err)
 
 	_, err = Serialize(aitem)
-	require.Error(t, err)
+	require.True(t, errors.Is(err, ErrTooBig), err)
 }