Bytes past Packet.meta.size are not valid to read from.
The commit makes the buffer field private and instead provides two
methods:
* Packet::data() which returns an immutable reference to the underlying
buffer up to Packet.meta.size. The rest of the buffer is not valid to
read from.
* Packet::buffer_mut() which returns a mutable reference to the entirety
of the underlying buffer to write into. The caller is responsible to
update Packet.meta.size after writing to the buffer.
880684565c