[ENHANCEMENT] Use AcceleratedArray (or something similar) as storage of named vertices

[mtfishman]

Currently we use NamedGraphs.OrderedDictionaries.OrderedIndices as the storage format for NamedGraph vertices. The idea is that it is a Dictionaries.AbstractIndices subtype, so it acts like a set where the indices and values are the members of the set. The OrderedIndices type adds on a separate interface layer where you can index into the OrderedIndices by integer position using NamedGraphs.OrdinalIndexing, so it acts both like a set and like an array.

I've found that this design works pretty well, but I think it causes some awkward situations in generic code that assumes collections are "array-like", i.e. are primarily indexed by integer offsets/positions. Probably we should switch out OrderedIndices for a type that still acts like a set in terms of hashing/fast lookup, but is primarily indexed by integer positions of elements of the set. The closest example I know of in the Julia ecosystem is AcceleratedArrays.jl, we could try that directly or role our own version if it doesn't suit our purposes directly.