Move internal/packed out of internal

[zenhack]

I recently ran into a use case where I wanted to using packed encoding on just a bare segment -- with no message header. The exposed interfaces in the main package don't provide a way to do this, and the raw Pack()/Unpack() apis in the packed package are under internal. I think we should move that package out of internal; there's no reason I see why this should be private.

[lthibault]

Oh good! I ran into a similar case a while back (I wanted to reuse []byte buffers ... can't remember the details), and this has been in the back of my mind ever since. +1 from me.