C# byte array `Count` instead of `Length`
Hi,
First of all many thanks for creating this, `kaitaistruct` saves me a lot of trouble building parsers. Second of all, sorry if this is a mistake on my part as I'm completely new to C# but it seems to me that for some reason the compiler uses a wrong property to refer to the length of a byte array.

```csharp
public Entry Payload
        {
            get
            {
                if (f_payload)
                    return _payload;
                long _pos = m_io.Pos;
                m_io.Seek(12);
                __raw_payload = m_io.ReadBytes(PayloadRaw.Count);
                var io___raw_payload = new KaitaiStream(__raw_payload);
                _payload = new Entry (io___raw_payload, this, m_root);
                m_io.Seek(_pos);
                f_payload = true;
                return _payload;
            }
        }
```

In the above code `PayloadRaw` does not have an attribute `Count`. When replaced to `Length` however, everything seems to be working fine. I can confirm that the problem appears when building a .NET Core app as well as a standard .NET dll file. The field this pertains to is an `instances` field:

```yaml
instances:
  payload:
    type: entry
    pos: 12
    size: payload_raw.size
```

Is my thinking correct that this should be easily fixable by modifying `TranslatorSpec`, `CSharpCompiler` and `CSharpTranslator` within `kaitaistruct-compiler`?