Move /src to /subprojects

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

subprojects/libutil/finally.hh

@@ -0,0 +1,47 @@
+#pragma once
+///@file
+
+#include <utility>
+#include <cassert>
+#include <exception>
+
+/**
+ * A trivial class to run a function at the end of a scope.
+ */
+template<typename Fn>
+class [[nodiscard("Finally values must be used")]] Finally
+{
+private:
+    Fn fun;
+    bool movedFrom = false;
+
+public:
+    Finally(Fn fun) : fun(std::move(fun)) { }
+    // Copying Finallys is definitely not a good idea and will cause them to be
+    // called twice.
+    Finally(Finally &other) = delete;
+    Finally(Finally &&other) : fun(std::move(other.fun)) {
+        other.movedFrom = true;
+    }
+    ~Finally() noexcept(false)
+    {
+        try {
+            if (!movedFrom)
+                fun();
+        } catch (...) {
+            // finally may only throw an exception if exception handling is not already
+            // in progress. if handling *is* in progress we have to return cleanly here
+            // but are still prohibited from doing so since eating the exception would,
+            // in almost all cases, mess up error handling even more. the only good way
+            // to handle this is to abort entirely and leave a message, so we'll assert
+            // (and rethrow anyway, just as a defense against possible NASSERT builds.)
+            if (std::uncaught_exceptions()) {
+                assert(false &&
+                    "Finally function threw an exception during exception handling. "
+                    "this is not what you want, please use some other methods (like "
+                    "std::promise or async) instead.");
+            }
+            throw;
+        }
+    }
+};