remake vector_set from inheritance from underlying container to aggre…

…gation

this makes it safer, as it disallows some of changes that could violate sorted contract
it also saves at least sizeof(void*) on most common comparators (due to empty struct optimization in compressed_pair)

con: it explicitly requires container class to follow vector API (almost all functions are exposed in vector_set)

to consider: remove non const front(), back(), at(), [] access.
Those are not safe either, left them only for compatibility