[boost] [containers] flat_set/flat_map could add insert_sorted(begin, end)

Hi Ion, I think the above member could be useful (taking an already sorted sequence), especially if the container needs to reallocate, in which case we may merge the two sequences [begin,end) [lower_bound(*begin),upper_bound(*(end-1))] and copy the unaltered prefix and suffix. Furthermore, even when we don't have to reallocate, we can bulk copy the suffix and simulate merging for the infix range. Any comments? -Thorsten