Implement first async message builder

src/main/kotlin/com/dmdirc/ktirc/IrcClient.kt

      * Sends the given command to the IRC server, and waits for a response back.
      *
      * This should only be needed to send raw/custom commands; standard messages can be sent using the
-     * extension methods in [com.dmdirc.ktirc.messages] such as TODO: sendJoinAsync.
+     * extension methods in [com.dmdirc.ktirc.messages] such as [com.dmdirc.ktirc.messages.sendPartAsync].
      *
      * This method will return immediately. The returned [Deferred] will eventually be populated with
      * the server's response. If the server supports the labeled-responses capability, a label will
      * Sends the given command to the IRC server, and waits for a response back.
      *
      * This should only be needed to send raw/custom commands; standard messages can be sent using the
-     * extension methods in [com.dmdirc.ktirc.messages] such as TODO: sendJoinAsync.
+     * extension methods in [com.dmdirc.ktirc.messages] such as [com.dmdirc.ktirc.messages.sendPartAsync].
      *
      * This method will return immediately. The returned [Deferred] will eventually be populated with
      * the server's response. If the server supports the labeled-responses capability, a label will

src/main/kotlin/com/dmdirc/ktirc/events/handlers/LabelledResponseHandler.kt → src/main/kotlin/com/dmdirc/ktirc/events/handlers/AsyncResponseHandler.kt

 
 import com.dmdirc.ktirc.IrcClient
 import com.dmdirc.ktirc.events.IrcEvent
+import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.launch
 
-internal class LabelledResponseHandler : EventHandler {
+internal class AsyncResponseHandler(private val scope: CoroutineScope = GlobalScope) : EventHandler {
 
     override fun processEvent(client: IrcClient, event: IrcEvent) {
         client.serverState.asyncResponseState.pendingResponses.values
                 .filter { it.second(event) }
                 .forEach {
-                    GlobalScope.launch {
+                    scope.launch {
                         it.first.send(event)
                     }
                 }

src/main/kotlin/com/dmdirc/ktirc/events/handlers/EventHandler.kt

         PingHandler(),
         ServerStateHandler(),
         UserStateHandler(),
-        LabelledResponseHandler()
+        AsyncResponseHandler()
 )

src/main/kotlin/com/dmdirc/ktirc/messages/MessageBuildersAsync.kt

+package com.dmdirc.ktirc.messages
+
+import com.dmdirc.ktirc.ExperimentalIrcClient
+import com.dmdirc.ktirc.events.ChannelParted
+
+/** Sends a request to part the given channel. */
+internal fun ExperimentalIrcClient.sendPartAsync(channel: String, reason: String? = null) =
+        sendAsync("PART", reason?.let { arrayOf(channel, reason) } ?: arrayOf(channel)) {
+            it is ChannelParted && isLocalUser(it.user) && it.target == channel
+        }