Need to call ac-niltree when pass list from scheme to arc, and more list->mlist is needed.
Export merge-list and replace use of destructive one.
Export sort-list, and use it to implement sort, instead of destructive
mergesort.
No need for r5rs now, as all stuff related to mutable pair in news.arc
has been removed.
Do not return at module level, so module import can be silence.
Update module path.