rewrite voice state handling

src/stores/VoiceStateStore.js

@@ -0,0 +1,20 @@
+const DataStore = require('./DataStore');
+const VoiceState = require('../structures/VoiceState');
+
+class VoiceStateStore extends DataStore {
+  constructor(guild, iterable) {
+    super(guild.client, iterable, VoiceState);
+    this.guild = guild;
+  }
+
+  add(data, cache = true) {
+    const existing = this.get(data.user_id);
+    if (existing) return existing;
+
+    const entry = new VoiceState(this.guild, data);
+    if (cache) this.set(data.user_id, entry);
+    return entry;
+  }
+}
+
+module.exports = VoiceStateStore;