`NodeMap` and other results need better reprs

[eriknw]

From #82, showing <graphblas_algorithms.classes.nodemap.NodeMap object at 0x00000206BC590550> is not the most helpful even though a NodeMap is a MutableMapping.

So, the repr of NodeMap should be updated to look more dict-like. Same for VectorMap, VectorNodeMap, NodeNodeMap, and NodeSet (these could probably use basic docstrings too!).