Create a wrapper around stdlib’s rename

Directly takes some c++ strings, and gently throws an exception on error
(rather than having to inline this logic everywhere)

src/libstore/local-binary-cache-store.cc

@@ -57,8 +57,7 @@ protected:
         AutoDelete del(tmp, false);
         StreamToSourceAdapter source(istream);
         writeFile(tmp, source);
-        if (rename(tmp.c_str(), path2.c_str()))
-            throw SysError("renaming '%1%' to '%2%'", tmp, path2);
+        moveFile(tmp, path2);
         del.cancel();
     }