Consistent terminology: Cell, Envelope, Message

This comes from discussions on arti#775 (closed) and arti!1839 (merged) . Briefly, we want to standardize our terminology so that we do not mix up different types of object, or get confused about what we are encoding and decoding.

Here is what I think we have settled on:

• A cell is always what we send over a channel. So for example, there can be a "CREATE2 cell" or a "NETINFO cell" or a "RELAY cell", but never a "BEGIN cell" or a "DATA cell" (since those are messages.)
  • A "RELAY cell" is one whose command field is 3 (that is, RELAY).
  • A "RELAY_EARLY cell" is one whose command field is 9 (that is, RELAY_EARLY).
  • A "relay cell" is a RELAY cell or a RELAY_EARLY cell.
• A message or relay message is always what we send over a circuit, encoded in a series of relay cells. We can say "BEGIN messages" and "DATA messages", but never "CREATE messages" or "NETINFO messages".
  • When we refer to a "message", we are not including its stream_id.
• When we want to refer to a message plus its (optional) stream_id, we call that an message envelope.

In terms of relationships:

• Some cells are relay cells. Some are not.
• All relay cells are used to encode message envelopes.
  • Before prop340, there was a 1:1 relationship between relay cells and message envelopes.
  • After prop340, there will be an m:n relationship between relay cells and message envelopes.
• Every message envelopes holds exactly one message. Some message envelopes additionally hold a stream_id.

I'll start making these edits after making sure that everybody agrees. I'll also rename the relevant objects in arti again.