[minor] reordering of the polymorphic set operations signature, to ease comparison / coherence with functorized operations
[minor] reordering of the polymorphic set operations signature, to ease comparison / coherence with functorized operations

It turns out that the following functions break consistency (missing
on one side or another, or under-specified in the polymorphic case):
compare, equal, map, filter_map, elements, pop, split, of_list.

bennn/batteries-included