Implement fromLabeledEdgeList

It was just stubbed out before.
Add blurbs for two adapters
Add a small helper to map labels to vertices.

This is useful while building a graph where each vertex label
corresponds to exactly one vertex.  The VertexMap efficiently
gets the vertex for each label, adding a new vertex if necessary.
Expand the VertexMap API, and use it

Replace some code implementing fromEdgeList that was doing
something similar.  This puts a common pattern in its own
module and exposes it for users.
Documentation, vertexMapFromGraph

Add a function to extract a VertexMap from an existing graph.
Return the vertex map from 'fromEdgeList'

We build it anyway, so we might as well at least let the user
use it if they want, rather than requiring them to rebuild it
with a linear pass.