Basic packet span length check

[bcronje]

With the new SizeField and other dynamic length methods, do you think its worth adding a very basic auto generated span length check property for the entire packet? Something like the following could be a nice to have little feature, especially when packets have dynamic length fields:

public bool HasValidSpanLength () => _span.Length <= GetTotalSize();

Or maybe even a generated TryParsePacket static method, where you want to create a packet over an existing packet buffer:

public static bool TryParse(Span<byte> packetData, out MyPacketSpan packet)
{
  var myPacket = new MyPacketSpan(packetData);
  if (myPacket.HasValidSpanLength())
  {
    packet = myPacket;
    return true;
  }
  packet = null;
  return false;
}

[enclave-alistair]

I like the idea; only thing to note is that the read/write operations assume that the underlying span has enough remaining data in it (relying on the Span<byte> to throw a "not enough data" exception, or whatever it is that gets thrown.

Also, the final 'TotalSize' expression contains all the size expressions in there; we almost want to check each 'dynamic' size expression in turn, and bail as soon as one of them exceeds the available length. That way we avoid ever invoking a size expression that refers to a point past the end of the buffer.