…iant name

…pes directly

…lder' into path-attributes-builder

…ssible

This commit adapts the codebase to use the types and traits introduced
in this branch. With that, several other concepts required changes, or
became deprecated and were removed.

The new types/traits include:
- trait AfiSafi, describing something that is characterized by a certain
  AFI+SAFI combination;
- trait AfiSafiNlri, an NLRI characterized by a certain AFI+SAFI
  combination;
- enum AfiSafiType, describing all AFI+SAFI combinations we support;
- enum Nlri<Octs>, now holding more explicit variants of dedicated
  types, a la Ipv4UnicastNlri, Ipv6UnicastNlri, etc.  For ADD-PATH,
  dedicated types are generated: Ipv4UnicastAddpathNlri, etc;
- On UpdateMessage, `fn typed_announcements()` provides an iterator
  generic over tpyes implementing AfiSafiParse, thus yielding NLRI of
  these dedicated types instead of enum variants (which is still
  available via the `fn announcements());
- UpdateBuilder is now generic over types that implement NlriCompose,
  making the adding/removing of announcements and withdrawals more
  straightforward. New generic helper methods include `fn
  add_announcements_from_pdu`.

Other changes:
- enum Safi is removed, as an SAFI by itself does not mean anything
  useful. We keep enum Afi and the new enum AfiSafiType. (The name
  `AfiSafi` is now used for the new trait);
- the UpdateMessage does not hold an entire SessionConfig anymore, but a
  trimmed down version of the new type PduParseInfo. The SessionConfig
  is only used in the very first call to `from_octets()` or `parse()`;
- MpReachNlri and MpUnreachNlri are not part of the PathAttribute enum
  anymore, we start treating them differently. Partly because with the
  new builders being generic, it would mean the enum PathAttribute must
  carry generic type info, and PaMap as well. All while we most often do
  not include the MP* attributes in those maps anyway;
- the new bgp::nlri modules replaces everyhing that was in
  bgp::message::nlri

Open issues / questions and other remarks:
- on UpdateMessage, we had several methods to specifically get unicast
  announcements. We need to figure out if those are still wanted;
- we need to figure out what the Workshop should look like. We can
  probably get rid of the afisafi_nlri module to a large extent;
- the UpdateBuilder now puts all withdrawals and announcements in MP
  attributes, so nothing goes into the conventional sections in the PDU.
  Related tests are ignored for now. We need to figure out to what
  extent we like to support those conventional sections in the builder.

…lder' into path-attributes-builder

…lk test in UpdateBuilder