[Networkx] Rewrite internals of `MixedEdgeGraph` to have a similar API, but simplified internals

[adam2392]

A lot of the mixedEdgeGraph internals was essentially copied over from networkx. I think with the assumption that "maybe" we could've kept similar internal data structures like _adj, _nodes, etc. Pretty much the only thing that works is _nodes because _adj is a very specific format for storing nested dicts. This issue is just to track what needs to eventually get rewritten for robustness.

Some of the existing code can be simplified because we don't have some of these underlying internals. Rather we let the networkx subgraphs deal w/ them. For example:

• subgraph does't work with subclasses