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/libexpr/parser.y

@@ -603,7 +603,7 @@ void EvalState::addToSearchPath(const string & s, bool warn)
     }
 
     if (isUri(path))
-        path = downloadFileCached(path, true);
+        path = downloadFileCached(store, path, true);
 
     path = absPath(path);
     if (pathExists(path)) {