Do not assume buffer.View boundaries for packet headers

`stack.PacketBuffer.Data` is documented to be guaranteed not to contain headers that split across views. However, after a discussion with bhaskerh, this guarantee is not true. As such, we should remove all assumptions about `buffer.View` boundaries for packet headers.

```
// Data holds the payload of the packet. For inbound packets, it also
// holds the headers, which are consumed as the packet moves up the
// stack. Headers are guaranteed not to be split across views.
//
// The bytes backing Data are immutable, but Data itself may be trimmed
// or otherwise modified.
Data buffer.VectorisedView
```