Implement a better GraphMap

[bluss]

We want to implement a better GraphMap, this has been expressed in several issues here, something that has about the current features but in a better and more featureful implementation.

Details to be worked out, it is not set in stone.

• Something that has key-lookup for nodes but still owns the node weights?
• Potentially have both node keys and node weights?
• More similar to Graph?
• Based on Indexmap?

[jasonwhite]

FYI, I wrote a hash table implementation similar to indexmap, but where removals don't perturb the indices: https://github.com/jasonwhite/holyhashmap

It's probably not very fast (it uses linear probing), but it works pretty well as the underlying data structure for a graph that owns its nodes.