Simplify AS_PATH encoding

Our current way of representing AS_PATH in the message and in RIB is overly complex, as it involves two nested unkeyed lists.

For compatibility reasons we need to retain this format, but introduce a simpler format, which will be authoritative.

It should include only two leaf-lists:

leaf-list as-sequence {
type inet:as-number;
ordered-by user;
}

leaf-list as-set {
type inet:as-number;
}

The parser needs to fill these out, concatenating segments in the message into these. On serialization, as-sequence entries need to be encoded first, 255 items at a time, followed by as-set entries.