Add edgeToVertexes function

[ajfriend]

It would be handy to have a function that provides the start and end vertices of an edge. I suppose such a function could work for both directed and (future) undirected edges. For the current directed edge mode, I think we would want the output vertices to be ordered according to the right hand rule:

The API might look something like these:

H3Error edgeToVertexes(H3Index edge, H3Index *v1, H3Index *v2);
H3Error edgeToVertexes(H3Index edge, H3Index *vertexes);