wkpool: Vertex Pool Topology for Well-Known Geometry

Establishes and maintains vertex pool topology for geometry handled by 'wk'. Segments are the atomic unit, vertices are shared via integer references into a pool. Topology is made discoverable via coincident vertex detection while not requiring modification of the input data. Topological data models follow principles described in Worboys and Duckham (2004, ISBN:978-0415283755). The edge-based topology geometry decomposed into vertices and directed edge pairs is a simplification of the quad-edge case in Guibas & Stolfi (1985) .