Use std::unique_ptr instead of g_autofree/g_autoptr()

This completes the transition of the code to std::unique_ptr so that we
can be assured that we use JS_free() on items that are acquired via
pass-by-reference via SpiderMonkey APIs, and thus we can remove more of
the g_free() calls on those as SpiderMonkey may switch to a different
allocation/deallocation implementation internally (which is currently
free()).

https://bugzilla.gnome.org/show_bug.cgi?id=777597