SpatialVector interface.

Idea: By doing vector calculations manually, you avoid the overhead of
building up and taking down genealized data structures like
PersistentVector. After all, we are not dealing with PersistentVectors,
we are dealing with 2ples (maybe 3ples in the future).

All thanks to @swannodette!
Refactoring using SpatialVector. Boids draw.