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/message/mod.rs

@@ -1,13 +1,14 @@
 #![allow(missing_docs)]
 
 mod core;
+mod disposal;
 mod events;
 mod mixer;
 mod udp_rx;
 mod udp_tx;
 mod ws;
 
-pub use self::{core::*, events::*, mixer::*, udp_rx::*, udp_tx::*, ws::*};
+pub use self::{core::*, disposal::*, events::*, mixer::*, udp_rx::*, udp_tx::*, ws::*};
 
 use flume::Sender;
 use tracing::info;