feature for supporting arena allocators

[GummyGun]

Worth pointing that this is mainly targeted towards the vulkan part of the crate. I believe that arena al locators are pretty useful specially in graphics(vulkan) when accessing some per level resources. right now this is kind of hard to with the current API, I have found some workarounds to achieve this style of resource acquisition but I think that it would be a good feature to give to the users of the crate. first of all, i think that this should be a opt-in since I understand that not all persons have the need for this kind of functionality. I was thinking of a method, that took Allocation and separated the memory part from a new struct that can be used for freeing the resource. at the same time I think that it would be useful for this functions to implement as well a unsafe_clone, since with arena style allocators resource counting is kind of redundant.

maybe even make the function that splits the Allocator into its parts a method on the Allocator

let my_allocation: Allocation = my_allocator.allocate(...)?; 
let (memory_part, freeing_part) = unsafe{my_allocator.defer_free(my_allocation)};
/*
memory_part can be safely binded to all kinds of resources. 
*/
my_allocator.free(freeing_part);
/*
memory_part shouldn't be accessed from here on out.  
*/

maybe some extra security could be further added in case that it is determined that it is needed. I hope that this call the attention and get consider for a further addition to this excellent crate, and of course I would be more than pleased to help in any way.