[boost] [interprocess] flat_set/flat_map should add front(), back(), operator[], pop_back()

Hi Ion, When using e.g. a flat_set as a priority queue, it is annoying not to have back() and pop_back() available. As for operator[](), then of couse cont.begin()[n] works, but this is somewhat less pretty compared to just cont[n]. Therefore I would really like to see all the above functions added to the interface. kind regards Thorsten