Change Device fields according to the spec

Fields `ip` and `last_seen` should be named `last_seen_ip` and
`last_seen_ts` according to the spec.

https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-devices

src/r0/device.rs

@@ -21,7 +21,7 @@ pub struct Device {
     pub display_name: Option<String>,
 
     /// Most recently seen IP address of the session.
-    pub ip: Option<String>,
+    pub last_seen_ip: Option<String>,
 
     /// Unix timestamp that the session was last active.
     #[serde(
@@ -29,5 +29,5 @@ pub struct Device {
         default,
         skip_serializing_if = "Option::is_none"
     )]
-    pub last_seen: Option<SystemTime>,
+    pub last_seen_ts: Option<SystemTime>,
 }