jid: Refactor all three JID types

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.

jid/Cargo.toml

@@ -19,6 +19,7 @@ edition = "2018"
 gitlab = { repository = "xmpp-rs/xmpp-rs" }
 
 [dependencies]
+memchr = "2.5"
 minidom = { version = "0.15", optional = true }
 serde = { version = "1.0", features = ["derive"], optional = true }
 stringprep = "0.1.2"