Multiple audio stream playback, volume control, pausing

* Fix Speaking state, use latest voice API version

* Speaking state would remain stuck on after playing particularly long
stretches of audio. So far as I can tell, playing 5 frames of silence
BEFORE changing the state seems to do the trick.
* Added new constant to make sure the library uses v3 of the voice api,
which it is written for.
* Heartbeat interval adjusted by * .75 as recommended by Discord.

* Initial version of new Audio wrapper.

* Single audio file case, as before..

* Loop over all available audio samples.

* Combine audio streams, account for volume.

* Cheaper explicit Opus silence frames.

As per Discord's recommendation, use a well-known 3-byte silence frame when needed.

* A bit of cleanup

Cleanup some of the code, rename some short-form fields to longer forms
(e.g. `s/src/source`), and remove a breaking change.

`Handler::play` was changed to return `LockedAudio` instead of `()`. If
someone were to rely on `Handler::play` returning `()`, the return type
change would break their code. Instead, this functionality has been
added to a new `Handler::play_returning` function.

1 files changed, 10 insertions, 3 deletions

diff --git a/src/voice/threading.rs b/src/voice/threading.rs
index 39b72ed..5731e86 100644
--- a/src/voice/threading.rs
+++ b/src/voice/threading.rs
@@ -15,7 +15,7 @@ pub(crate) fn start(guild_id: GuildId, rx: MpscReceiver<Status>) {
 }
 
 fn runner(rx: &MpscReceiver<Status>) {
-    let mut sender = None;
+    let mut senders = Vec::new();
     let mut receiver = None;
     let mut connection = None;
     let mut timer = Timer::new(20);
@@ -40,7 +40,14 @@ fn runner(rx: &MpscReceiver<Status>) {
                     receiver = r;
                 },
                 Ok(Status::SetSender(s)) => {
-                    sender = s;
+                    senders.clear();
+
+                    if let Some(aud) = s {
+                        senders.push(aud);
+                    }
+                },
+                Ok(Status::AddSender(s)) => {
+                    senders.push(s);
                 },
                 Err(TryRecvError::Empty) => {
                     // If we receieved nothing, then we can perform an update.
@@ -62,7 +69,7 @@ fn runner(rx: &MpscReceiver<Status>) {
         // another event.
         let error = match connection.as_mut() {
             Some(connection) => {
-                let cycle = connection.cycle(&mut sender, &mut receiver, &mut timer);
+                let cycle = connection.cycle(&mut senders, &mut receiver, &mut timer);
 
                 match cycle {
                     Ok(()) => false,