Clean up graph subpackage

[viking-sudo-rm]

The structure of the code could be cleaner:

1 Flatten graph::avl_graph subpackage into graph
2. Move graph::memory_backing to memory_backing
3. Generics order for Dawg<E, N> should match Graph<N, E> (and rename W -> N)

There are probably lots of other opportunities to improve readability

[viking-sudo-rm]

Also, there is now a weird situation in the design where MemoryBacking as a type specifies the memory backing of the graph, but mb: MemoryBacking acts as like a config for the graph?

Could probably be refactored/simplified