memory: Add allocator_traits and deprecate old specialized version

[stotko]

The containers use default_allocator_traits as well as the allocate() and deallocate() functions of its allocator directly. Although this approach was sufficient to relax several restrictions, it is inconsistent and does not reflect the desired behavior specified in the C++ standard. Introduce allocator_traits<Allocator> which resolves this problem and provide a standardized access the allocators. Deprecate the old default_allocator_traits which do not model the interface correctly. Note that the current implementation is oversimplified and always uses the fallback solution rather than detecting the capabilities of Allocator. Furthermore, this fixes an attribute issue in the recently introduced get_allocator() functions (see #56).