More efficient memory access for `DiskBacking`

[viking-sudo-rm]

Right now, whenever we read/write a field on a node, we read/write the whole thing rather than just part of it.

However, a lot of the code just accesses individual fields (like the failure transition) at a time. It would be nice to change it so that we only read/write the field we need. There are maybe two ways to do this:

1. Only read specific byte indices from a Node or Edge struct stored in a vector of nodes and edges
2. Separate out the different attributes of nodes and edges into different files (lengths.vec, failures.vec, etc.) and accessing elements of those.