Fix double destroy of top node in skew_heap and simplify allocator usage

[glenfe]

@timblechmann the changes are:

• In the first commit:
  • Fix the double-destroy of the top node in skew_heap (you can see the destructor being called and then called again).
• In the second commit:
  • For allocate(n) and deallocate(p, n) call them directly. Every allocator is required to provide them. Only allocate(n, hint) needs special handling (e.g. using allocator_traits<A>::allocate(a, n, hint) or boost::allocator_allocate(a, n, hint).
  • For construction and destruction because you're constructing nodes not value_type, use placement new. For something like std::list<T> you'll see that the node types are not constructed using the allocator, but instead the T inside the node is constructed using the allocator.
  • Use the allocator access utilities instead of having #ifdef BOOST_NO_CXX11_ALLOCATOR blocks everywhere.

[timblechmann]

looks all good, thanks a lot!