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/mod.rs

@@ -1,5 +1,6 @@
 #![allow(missing_docs)]
 
+pub(crate) mod disposal;
 pub mod error;
 mod events;
 pub mod message;