Use a type family and GAT to simplify vertex entity memory schemes.

[hadronized]

Instead of using VertexEntity<V, …, Interleaved, …>, one can now use something like VertexEntity<V, …, Interleaving, …>. The way it works is by implementing a new trait, VertexStorageFamily, which has an associated type (type family) using a GAT:

type Storage: AsVertexStorage;