Input & Driver: Fix zombie processes on Unix (#39)

Linux/Unix requires that processes be waited, which is unfortunate as Windows lets us abandon them to the murderous whims of the OS. This PR adds Unix-specific behaviour to send a SIGINT before waiting on the process, and adds an additional thread per call for asset disposal on all platforms.

Closes #38.

---

* Close processes by SIGINT and wait on Unix

This seems to remedy the Linux-specific zombie processes. Addition of
nix as a dependency *should* be fine on Windows, since I believe it
compiles to an empty crate.

* Dispose of Tracks on auxiliary thread

This adds a mechanism for the mixer threads to perform potentially expensive deallocation/cleanup outside of the main loop, preventing deadline misses etc. This should make misbehaving `wait`s a bit more friendly.

src/driver/tasks/disposal.rs

@@ -0,0 +1,18 @@
+use super::message::*;
+use flume::Receiver;
+use tracing::instrument;
+
+/// The mixer's disposal thread is also synchronous, due to tracks,
+/// inputs, etc. being based on synchronous I/O.
+///
+/// The mixer uses this to offload heavy and expensive drop operations
+/// to prevent deadline misses.
+#[instrument(skip(mix_rx))]
+pub(crate) fn runner(mix_rx: Receiver<DisposalMessage>) {
+    loop {
+        match mix_rx.recv() {
+            Err(_) | Ok(DisposalMessage::Poison) => break,
+            _ => {},
+        }
+    }
+}