Use util::allocator in swapchain object
Change util::allocator's create and destroy methods so that they can allocate multiple objects, in line with the allocate and deallocate methods of std::allocator.
Also add documentation for util::allocator and its methods.
Finally, use util::allocator in swapchain.cpp, rather than allocating memory directly via VkAllocationCallbacks.
Change-Id: I0bc25abe3cbc3af9608218411da8d70e04dd9749 Signed-off-by: Matteo Franchin matteo.franchin@arm.com