Add g_autofree

Add g_autofree

The g_autoptr() being associated with the type name works out really
well for things like GHashTable.  However, it's a bit more awkward to
associate with "gchar".  Also because one can't use "char".
Similarly, there are a lot of other "bare primitive array" types that
one might reasonably use.

This patch does not remove the autoptr for "gchar", even though I
think it's rather awkward and strange.

Also while we're here, add a test case for the cleanup bits.

https://bugzilla.gnome.org/show_bug.cgi?id=744747