Voice Rework -- Events, Track Queues (#806)

This implements a proof-of-concept for an improved audio frontend. The largest change is the introduction of events and event handling: both by time elapsed and by track events, such as ending or looping. Following on from this, the library now includes a basic, event-driven track queue system (which people seem to ask for unusually often). A new sample, `examples/13_voice_events`, demonstrates both the `TrackQueue` system and some basic events via the `~queue` and `~play_fade` commands.

Locks are removed from around the control of `Audio` objects, which should allow the backend to be moved to a more granular futures-based backend solution in a cleaner way.

src/input/child.rs

@@ -0,0 +1,38 @@
+use super::*;
+use std::{
+    io::{BufReader, Read},
+    process::Child,
+};
+use tracing::debug;
+
+/// Handle for a child process which ensures that any subprocesses are properly closed
+/// on drop.
+#[derive(Debug)]
+pub struct ChildContainer(Child);
+
+pub(crate) fn child_to_reader<T>(child: Child) -> Reader {
+    Reader::Pipe(BufReader::with_capacity(
+        STEREO_FRAME_SIZE * mem::size_of::<T>() * CHILD_BUFFER_LEN,
+        ChildContainer(child),
+    ))
+}
+
+impl From<Child> for Reader {
+    fn from(container: Child) -> Self {
+        child_to_reader::<f32>(container)
+    }
+}
+
+impl Read for ChildContainer {
+    fn read(&mut self, buffer: &mut [u8]) -> IoResult<usize> {
+        self.0.stdout.as_mut().unwrap().read(buffer)
+    }
+}
+
+impl Drop for ChildContainer {
+    fn drop(&mut self) {
+        if let Err(e) = self.0.kill() {
+            debug!("Error awaiting child process: {:?}", e);
+        }
+    }
+}