Also added networkBytesAsValue and restored bytesAsValue.
These are useful for treating the bytes from the network directly as a Message.
Otherwise, the init function would overwrite the packet type and length to be correct.
I would like the message handling to fail if the message body is incorrect.
The slice sets us avoid allocating within the init function.
This means init can't fail, and it also makes it easier to stack allocate messages (slice an array buffer, instead of creating a stack allocator).
