Eliminate the "store" global variable

Also, move a few free-standing functions into StoreAPI and Derivation.

Also, introduce a non-nullable smart pointer, ref<T>, which is just a
wrapper around std::shared_ptr ensuring that the pointer is never
null. (For reference-counted values, this is better than passing a
"T&", because the latter doesn't maintain the refcount. Usually, the
caller will have a shared_ptr keeping the value alive, but that's not
always the case, e.g., when passing a reference to a std::thread via
std::bind.)

src/nix-prefetch-url/nix-prefetch-url.cc

@@ -91,8 +91,8 @@ int main(int argc, char * * argv)
         if (args.size() > 2)
             throw UsageError("too many arguments");
 
-        store = openStore();
-        EvalState state(searchPath);
+        auto store = openStore();
+        EvalState state(searchPath, store);
 
         Bindings & autoArgs(*evalAutoArgs(state, autoArgs_));