The main reason for this refactor was to make common operations simpler,
for instance formatting a JID is now a simple clone of a String.
Instead of having three different String for each of node, domain and
resource, we now have a single String with offsets pointing to where the
at and slash are (if they are present).
This also reduces the size of a FullJid from 72 bytes to 32 bytes on
64-bit platforms (less so on 32-bit), and BareJid from 48 bytes to
32 bytes. Jid is still 40 bytes instead of 32, but that can be improved
in a future version where InnerJid has been inlined into each struct.
