Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/exit.hh

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <exception>
+
+namespace nix {
+
+/**
+ * Exit the program with a given exit code.
+ */
+class Exit : public std::exception
+{
+public:
+    int status;
+    Exit() : status(0) { }
+    explicit Exit(int status) : status(status) { }
+    virtual ~Exit();
+};
+
+}