Eliminate the substituter mechanism

Substitution is now simply a Store -> Store copy operation, most
typically from BinaryCacheStore to LocalStore.

src/libutil/finally.hh

@@ -0,0 +1,12 @@
+#pragma once
+
+/* A trivial class to run a function at the end of a scope. */
+class Finally
+{
+private:
+    std::function<void()> fun;
+
+public:
+    Finally(std::function<void()> fun) : fun(fun) { }
+    ~Finally() { fun(); }
+};