This additionally should optimize the parsing a tiny bit by looking for
both @ and / at the same time and iterating on them, instead of one by
one.
Thanks nicoco for the report!
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.
