Resume on resumable session invalidations

Session invalidations include whether the session may be resumable.
Previously, this was not parsed by serenity. This data is now contained
in `GatewayEvent::InvalidateSession` as a boolean, which constitutes a
breaking change for users that manually handle gateway events.

Upgrade path:

Instead of pattern matching on simply the variant like so:

```rust
use serenity::model::event::GatewayEvent;

match gateway_event {
    GatewayEvent::InvalidateSession => {
        // work here
    },
    // other matching arms here
    _ => {},
}
```

Instead pattern match it as a single field tuple-struct:

```rust
use serenity::model::event::GatewayEvent;

match gateway_event {
    GatewayEvent::InvalidateSession(resumable) => {
        // work here
    },
    // other matching arms here
    _ => {},
}
```

src/gateway/shard.rs

@@ -434,16 +434,17 @@ impl Shard {
                     self.autoreconnect().and(Ok(None))
                 }
             },
-            Ok(GatewayEvent::InvalidateSession) => {
+            Ok(GatewayEvent::InvalidateSession(resumable)) => {
                 info!(
-                    "[Shard {:?}] Received session invalidation; re-identifying",
-                    self.shard_info
+                    "[Shard {:?}] Received session invalidation",
+                    self.shard_info,
                 );
 
-                self.seq = 0;
-                self.session_id = None;
-
-                self.identify().and(Ok(None))
+                if resumable {
+                    self.resume().and(Ok(None))
+                } else {
+                    self.identify().and(Ok(None))
+                }
             },
             Ok(GatewayEvent::Reconnect) => self.reconnect().and(Ok(None)),
             Err(Error::Gateway(GatewayError::Closed(data))) => {
@@ -981,6 +982,7 @@ impl Shard {
         self.heartbeat_instants = (Some(Instant::now()), None);
         self.heartbeat_interval = None;
         self.last_heartbeat_acknowledged = true;
+        self.session_id = None;
         self.stage = ConnectionStage::Disconnected;
         self.seq = 0;
     }

src/model/event.rs

@@ -1060,7 +1060,8 @@ pub enum GatewayEvent {
     Dispatch(u64, Event),
     Heartbeat(u64),
     Reconnect,
-    InvalidateSession,
+    /// Whether the session can be resumed.
+    InvalidateSession(bool),
     Hello(u64),
     HeartbeatAck,
 }
@@ -1096,7 +1097,15 @@ impl GatewayEvent {
                 GatewayEvent::Heartbeat(s)
             },
             OpCode::Reconnect => GatewayEvent::Reconnect,
-            OpCode::InvalidSession => GatewayEvent::InvalidateSession,
+            OpCode::InvalidSession => {
+                let resumable = map.remove("d")
+                    .ok_or_else(|| {
+                        DeError::custom("expected gateway invalid session d")
+                    })
+                    .and_then(bool::deserialize)?;
+
+                GatewayEvent::InvalidateSession(resumable)
+            },
             OpCode::Hello => {
                 let mut d = map.remove("d")
                     .ok_or_else(|| DeError::custom("expected gateway hello d"))