Fallback on integer vertices

[gdalle]

On Discourse, the question arose of how to combine generic vertices with efficient storage.

@CameronBieganek suggested interfaces methods to translate between vertices and integers. This is similar to what many package developers have done for graphs with metadata.
@mtfishman says we should look at what array packages like DimensionalData.jl and AxisKeys.jl have done, by similarity between a graph and a matrix.