Data Structure implementations.

[rphmeier]

Having the ability to allocate singular values is nice, but for users to get optimal use of allocators, they will undoubtedly want to have at the very least a Vec, and more likely even more data structures. It's unfortunate that those in the standard library aren't generic over the allocator like those in the c++ stl, since the interfaces will probably be either identical or very similar to facilitate ease-of-use. The only difference is that mutating calls that will reallocate have to be allowed to fail.

With that said, should these implementations reside in this crate or in another supplementary crate? Or, as a third option, users could be expected to implement them themselves as-needed.