Get rid of std::shared_ptr<std::string> and ref<std::string>

These were needed back in the pre-C++11 era because we didn't have
move semantics. But now we do.

src/libstore/nar-accessor.hh

@@ -10,7 +10,7 @@ struct Source;
 
 /* Return an object that provides access to the contents of a NAR
    file. */
-ref<FSAccessor> makeNarAccessor(ref<const std::string> nar);
+ref<FSAccessor> makeNarAccessor(std::string && nar);
 
 ref<FSAccessor> makeNarAccessor(Source & source);